Tampere University of Technology

Self-Regulating Iris Based on Light-Actuated Liquid Crystal Elastomer

CitationZeng H Wani O M Wasylczyk P Kaczmarek R amp Priimaumlgi A (2017) Self-Regulating Iris Based on Light-Actuated Liquid Crystal Elastomer Advanced Materials httpsdoiorg101002adma201701814

Year2017

VersionPeer reviewed version (post-print)

Link to publicationTUTCRIS Portal (httpwwwtutfitutcris)

Published inAdvanced Materials

DOI101002adma201701814

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Download date16012021

1

NOTE This is the peer reviewed version of the following article

bdquoSelf-regulating iris based on light-actuated liquid crystal

elastomerldquo

which has been published in final form at

[httpdoiorg101002adma201701814]

This article may be used for non-commercial purposes in

accordance with Wiley Terms and Conditions for Use of Self-

Archived Versions

2

DOI 101002((please add manuscript number))

Article type Communication

Self-regulating iris based on light-actuated liquid crystal elastomer

Hao Zeng Owies M Wani Piotr Wasylczyk Radosław Kaczmarek Arri Priimagi

Dr H Zeng O M Wani Prof A Priimagi

Laboratory of Chemistry and Bioengineering Tampere University of Technology PO Box

541 FI 33101 Tampere Finland

E-mail haozengtutfi arripriimagitutfi

Dr P Wasylczyk

Photonic Nanostructure Facility Institute of Experimental Physics Faculty of Physics

University of Warsaw ul Pasteura 5 02-093 Warsaw Poland

Dr R Kaczmarek

Department and Clinic of Ophthalmology Wrocław Medical University ul Borowska 213

50-556 Wrocław Poland

Keywords (iris biomimetic photoactuation liquid crystal elastomer azobenzene)

Abstract Iris found in many animal species is a biological tissue that can change the

aperture (pupil) size to regulate light transmission into the eye in response to varying

illumination conditions The self-regulation of the eye lies behind its auto-focusing ability and

large dynamic range rendering it the ultimate ldquoimaging devicerdquo and a continuous source of

inspiration in science In optical imaging devices adjustable apertures play a vital role as they

control the light exposure the depth of field and optical aberrations of the systems Tunable

irises demonstrated to date require external control through mechanical actuation and are not

capable of autonomous action in response to changing light intensity without control circuitry

A self-regulating artificial iris would offer new opportunities for device automation and

stabilization Here we report the first iris-like liquid-crystal elastomer device that can

perform automatic shape-adjustment by reacting to the incident light power density Similar to

natural iris the device closes under increasing light intensity and upon reaching the minimum

3

pupil size reduces the light transmission by a factor of 7 The light-responsive materials

design together with photoalignment-based control over the molecular orientation provide a

new approach to automatic self-regulating optical systems based on soft smart materials

Depending on biological species pupils come in diverse geometries comprising ellipses slits

pin-holes and crescent shapes helping animals to adapt to the specific requirements of their

living environment[1] In human iris the sphincter and dilator muscles control the pupil size in

order to stabilize the light transmission into the retina[2] In all cases a disturbance in the iris

functionality leads to severe visual dysfunctions Just as in humananimal eyes adjustable

apertures are vital in technological applications particularly in imaging systems and

spectrometers Photographic devices commonly contain a mechanical iris diaphragm

consisting of several thin segments that form an adjustable polygonal aperture which allows

controlling the depth of field optical aberrations and other parameters that influence the

image quality[3] Spectrometers implement tunable slits to control the resolution and to adjust

the overall signal level during data collection[4] Recently compact and tunable aperture

devices have been made using different actuation mechanisms eg electromagnetic

heating[5] electro-wetting in micro-fluidics[67] and piezo-mechanics[8] All these iris-like

devices are passive in the sense that they require external control (ie a separate controller

apart from the iris) and ex-situ light detection schemes An iris device that can self-adjust the

aperture size in response to incident light intensity would provide access to automated optical

systems in environments where no controlling circuits can access eg opto-fluidic channels

However their technical realization remains a grand challenge

The only way to realize an iris device that senses the light field is to fabricate it from a

material that itself is photo-responsive and devise a configuration that openscloses in

response to decreasingincreasing light levels As we show such actuation can be reached in

light-responsive liquid-crystal elastomers (LCEs) which are smart materials that can exhibit

4

large shape deformation upon illumination[9-11] Light-responsive LCEs comprise a loosely

crosslinked network of liquid-crystalline moieties with controlled molecular alignment

whereas the light-active units can be either part of the polymer network[12] or simply doped

into it[13] The elastic network can undergo reversible shape changes in response to

photochemical[9] (eg trans-cis isomerization) or photothermal[14] stimulus In either case the

benefit of the light-based actuation scheme relies on remote non-contact energy supply and

the precise spatial and temporal control over light properties Several LCE-based soft-robotic

devices have been reported exploiting these features such as micro-swimmers[1516]

walkers[1718] and transportation devices[19] Self-regulating behavior in light-driven LCE film

has been demonstrated in simple photomechanical oscillating systems[20] and recently in an

artificial flytrap device capable of autonomous action based on optical feedback[21]

Herein we report an LCE iris that can autonomously open and close by responding to

the incident light intensity without any need for external control circuitry Mimicking the

properties of a natural iris (Figure 1ab) the device openscloses upon decreasingincreasing

light levels To accomplish the first goal we use LCE-based soft actuator and light-induced

bending as the mechanism responsible for the modification of the aperture size To fulfil the

second point we implement photoalignment-based control over molecular alignment to

achieve radial molecular orientation within the LCE film and fabricate a structure with 12

radially aligned segments (Figure 1cd) We employ anisotropic thermal expansion within the

polymerized LCE to maintain the iris in the open state under low-light conditions while at

increased exposures light-triggered molecular reorientation forces the iris to close and thus to

reduce the aperture size To the best of our knowledge this is the first demonstration of

artificial iris that is both powered and controlled with light performing transmission

adjustment relying directly on external light intensity

Bending of a thin strip is the most common way to characterize liquid-crystal polymer

actuation under external stimuli[22-25] From the technological point of view it is often most

5

convenient to fabricate an LCE film in a glass cell with thickness d ranging from a few to a

few tens of m and length l of several millimetres In such films bending upon light

absorption is due to the strain difference ∆ε occurring between the two surfaces with a simple

relationship between the bending angle α and the geometry α ∆εld (l gtgtd) Such bending

film can be extremely sensitive for d = 10 μm and l = 5 mm ∆ε as low as 016 may be

sufficient to induce a bending of 90deg Due to this high sensitivity changes in the bending

angle are affected by many parameters such as environmental variations inhomogeneities in

the material composition or imperfections in the film fabrication process It is widely

accepted that the initial bending existing even when the film is not exposed to light is due to

residual stress in the LC network generated during photopolymerization[2627] The nature of

the residual stress is complex depending delicately on the molecular system used Apart from

uniform shrinkage[28] other reasons for building up the residual stress during polymerization

could be for instance opposite diffusion of monoacrylate and diacrylate monomers during

photopolymerization[29] and cis-to-trans isomerization in the darkness after

photopolymerization[26] However the initial bending phenomenon has rarely been connected

with anisotropic thermal expansion[3031] and to the best of our knowledge it has not been

previously utilized in soft actuator design

Liquid crystal elastomers are anisotropic materials exhibiting not only optical

birefringence but also anisotropic thermal expansion governed by molecular orientation[2332]

This is pertinent when the photopolymerization is performed at elevated temperatures in order

to stabilize the LC phase Figure 2a illustrates an LCE film polymerized at temperature Tp

which is higher than the room temperature TR When a strip is cut out from the film it remains

flat only when the environmental temperature Te is equal to Tp After polymerization however

the actuation experiments are typically carried out at room temperature (in our case TR =

22 degC) tens of degC below the Tp In a splay-aligned film different thermal expansion at

different film surfaces will create the stress and thus induce spontaneous bending of the film

6

When the same film is polymerized at TR (Tp = TR) no stress is preserved inside the material

at TR and no spontaneous bending occurs However when Te is elevated the strip bends to the

opposite direction as illustrated in Figure 2b To demonstrate the utility of the anisotropic

thermal expansion we prepared a series of 20 μm thick splay-aligned LCE strips

polymerized them at different temperatures and monitored the bending angles α (upon no

light irradiation) at different temperatures As shown in Figure 2c an LCE strip polymerized

at 40 degC (solid pink dot) shows gradually increasing bending deformation upon slow cooling

reaching the maximum bending at TR A strip polymerized at 20 degC (red dots) remains flat at

TR In Figure 2d we plot the α vs temperature difference ∆T between polymerization and

room temperature showing that for ∆T gt 0 ie when the polymerization temperature is

higher than the operation temperature maximum initial bending occurs when the

polymerization temperature is 20-30 degC higher than the operation temperature With the

photopolymerizable mixture we employed further increase in the polymerization temperature

to 60 degC (∆T = 38 degC) brings the mixture close to the LC-to-isotropic phase transition

temperature Tc = 64 degC in which case even slight absorption from the polymerizing light can

trigger partial phase transition reduce the order parameter and thus decrease the bending

angle of the LCE strip at TR We note that this initial bending (for ∆T gt 0) is in the direction

opposite to the light-induced bending Therefore we can make use of it in our artificial iris

design (Figure 1cd) making it open in the dark and closed upon light illumination which is

opposite to ldquoconventionalrdquo commonly used light-induced bending in LCE films

To realize the LCE iris it is imperative that all the bending segments actuate

symmetrically towards the same central point requiring the LCE constituents to be radially

aligned Obtaining such alignment using conventional preparation procedures such as rubbing

and mask-exposure[173334] is challenging hence we used photoalignment technique[35-37] to

achieve the desired molecular alignment The sample preparation procedure is illustrated in

Figure 3 The LC cell (20 μm thick) was constructed such that at the upper surface the LC

7

molecules adopted homeotropic alignment while the bottom surface was coated with an

azobenzene-based photoalignment layer (PAAD-72 BEAM Co) allowing us to define the

LC alignment with polarization of the incident light (Figure 3bc) To obtain the radial

molecular alignment we used a radial-polarization converter[38] to transform linearly

polarized light into azimuthally polarized as illustrated in Figure 3d The molecules within

the photoalignment layer orient themselves in the direction perpendicular to the polarization

of the incident light hence azimuthal polarization gives rise to radial photoalignment (Figure

3cd) and consequently radial alignment of the photopolymerizable LC mixture close to the

bottom surface of the cell To make the mixture responsive to visible light (we wanted to

avoid using UV light in order to obtain ldquohuman-friendlyldquo action) we doped it with Disperse

Red 1 (DR1 4 mol-) Upon irradiation with wavelengths in the bluendashgreen region of the

spectrum DR1 is known to undergo continuous trans-cis-trans cycling thereby releasing a

significant amount of heat It is the photothermal heating that we employ in our actuator

design The chemical structures of the compounds used in this work are shown in Figure 3e

The polymerization was carried out at 45 degC in order to maximize the initial bending (Figure

2) After polymerization the material has a Youngrsquos modulus of 25 MPa at room temperature

(Figure S1) and its glass transition temperature is below room temperature as confirmed by

the differential scanning calorimetry curves shown in Figure S2 The cross-polarized optical

micrograph of the LCE film shown in the inset of Figure 3f revealed that the radial

molecular alignment was preserved after polymerization After opening the cell a disc with

140 mm diameter (approximately the size of the human iris) with 12 petal-shape segments

was cut out (Figure 3f) After releasing the stress by annealing at 50 degC the device

spontaneously ldquoopened uprdquo and adapted a flower-like shape at room temperature with a

central hole of about 10 mm in diameter (Figure 3g) In this geometry each ldquopetalrdquo segment

is able to deform in response to the local light intensity as shown in Movie 1 where the LCE

segments move in response to a scanned light beam Upon uniform illumination with 470 nm

8

LED light all iris segments bend inwards and the aperture closes (Figure 3h Movie 2) For

more details on the fabrication process we direct the reader to the Supporting Information

To characterize the performance of the device a 488 nm laser beam with a diameter of

12 mm was projected onto the iris and the transmitted light as a function of the incident light

power was measured (Figure 4a) When the incident power was slowly increased from 20 to

120 mW all segments deformed inwards and gradually reduced the aperture size For laser

powers above 120 mW some segments closed completely resulting in a rapid decrease in the

iris transmission At around 180 mW all the segments bent inwards and the aperture closed

reaching transmission as low as 10 The self-regulating iris is able to adjust the transmitted

light power across a 200 mW range approximately 20 mW of light is transmitted for both 30

mW input (corresponding to 70 transmission) and 200 mW input (10 transmission red

line in Figure 4a) Figure 4b shows the iris response dynamics after the device is illuminated

instantaneously with 270 mWcm-2 intensity and the output power through the iris is recorded

The decreasing curve of the output power comprises several steps each of them

corresponding to one or few iris segments closing Differences in the closure time for

different segments are attributed to fabrication defects (eg slight variation of thickness and

molecular alignment between segments) and high sensitivity of the bending actuation Images

of the iris at different stages of the closing action are shown in the insets of Figure 4b

revealing the separated closed segments and asymmetric shape of the aperture at the

intermediate stages The asymmetric shape of the aperture does not harm the image

processing in photographic application but it may offer novel optical functionalities The

diversity in the material design and the versatility of the photoalignment-based patterning

technique[39] also provide the opportunities to realize artificial irises with different shapes and

photodeformation modes mimicking the diverse iris geometries observed for different animal

species[1] Real-time closing action is shown in Movie 3 where an iris is placed in front of a

LED light source and responds to blocking the light beam Figure 4c plots the closure times

9

for the 12 individual segments under different incident light intensities Upon low-intensity

excitation the closure time varies significantly for the different segments whereas the

variation is much smaller at high light intensities The statistical analysis of the closure times

is shown in Figure S3 of the Supporting Information The complete closure action takes place

in 30 s upon 230 mW cm-2 incident light intensity and in less than 5 s upon 300 mW cm-2

irradiation

We attribute the closing action of the iris mostly to photothermal heating of the LCE

film[14] The heating is demonstrated in Movie 4 showing the light-induced temperature

increase of the LCE film imaged with an infrared camera revealing that the iris becomes flat

(all segments closed) when the maximum temperature within the segments reaches 49 degC

This value is consistent with the polymerization temperature (45 degC) in which case no initial

bending due to the anisotropic thermal expansion is expected (see the pink dots in Figure 2c)

Different from photochemical (isomerization-induced) actuation that is strongly influenced by

optical density[9] the light-induced temperature increase is insensitive to light penetration

depth into the material During photothermal actuation both material surfaces experience

similar temperature increase due to fast heat conduction across the relatively thin film The

temperature-dependent actuation properties allow us to also obtain an iris device with

opposite action ie one that is closed in the dark and opens upon light irradiation This is

demonstrated in Movie 5 showing an iris deforming on a hot plate at 50 degC closing up

without light and opening again upon illumination As an additional benefit the system can be

made sensitive to different wavelengths of light simply by changing the dye inducing the

photothermal effect while sensitivity over broader spectral range can be obtained by doping

the system with several dyes or broadband absorbers We note that the dyes do not have to be

azobenzenes[14] as we believe photoisomerization to play only a minor role in the

performance of the iris However if photochemical actuation was implemented instead of

light-induced heating the iris functionality could be obtained also in liquid environment

10

paving way towards optofluidic applications or actuation in biologically relevant

environments

To conclude we report what we believe to be the first artificial iris that can self-

regulate the aperture size in direct response to the incident light intensity variations We

employ anisotropic thermal expansion of LCEs in order to maintain the iris in the open state

in the dark while radial molecular alignment was used to induce circularly symmetric

actuation of the iris segments thus closing the aperture upon illumination with visible light

The diversity of material design and sophisticated form of actuation paves way towards

adjustable soft optics with light-control capacity and offers alternatives to realize micro-

robotic systems with smart features such as self-recognition stabilization and automatic

manipulation

Experimental Section

Materials The LCE actuator is made by polymerization of a mixture that contains 75 mol

of LC monomer 4-Methoxybenzoic acid 4-(6-acryloyloxyhexyloxy)phenyl ester 20 mol of

LC crosslinker 14-Bis-[4-(3-acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene (both

purchased from Synthon chemicals) 4 mol of light-responsive molecule N-Ethyl-N-(2-

hydroxyethyl)-4-(4-nitrophenylazo)aniline (Disperse Red 1 Sigma Aldrich) and 1 mol of

photo-initiator (22-Dimethoxy-2-phenylacetophenone Sigma Aldrich) All compounds were

used as received For sample preparation details see the Supporting Information

Characterization Absorption spectra were measured with a custom-modified UV-Vis

spectrophotometer (Cary 60 UV-Vis Agilent Technologies) with a polarization controller

Optical images and movies were recorded by using a Canon 5D Mark III camera (f = 100 mm

lens) and thermal images were recorded with an infrared camera (FLIR T420BX close-up

2timeslens 50 μm resolution) Spatially filtered light from a continuous-wave laser (488 nm

Coherent Genesis CX SLM) was expanded to a diameter of 12 mm and controlled with a

11

beam shutter before impinging on the soft iris The light impinged onto the iris always from

the side with bent segments in order to avoid shadowing by the exterior sections of the iris

The power of incident and transmitted light were measured with a microscope-slide power

sensor (Thorlabs S170C) For the iris actuation demonstrations (Movies 1 2 3 5) an LED

(Prior Scientific 470 nm) with tunable output power was used for light actuation For

temperature-dependent bending angle measurements LCE strips were immersed in a water

bath on a heating stage An infrared camera was used to monitor the water temperature and

the images at different water temperatures were captured using the Canon camera in the top

view No sensitivity to humidity was found in the LCE For measuring the mechanical

properties a planar-aligned LCE sample with dimensions 10 times 2 times 005 mm3 was cut along

the nematic director and vertically attached to a force sensor and a linear motor stage The

stressndashstrain curve was measured at room temperature Differential scanning calorimetry

measurement was performed with a Mettler Toledo Star DSC821e instrument using

heatingcooling speeds of 10 degC min-1

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author

Acknowledgements

A P gratefully acknowledges the financial support of the European Research Council

(Starting Grant project PHOTOTUNE Agreement No 679646) O W is thankful to the

graduate school of Tampere University of Technology (TUT) and H Z to the TUT

postdoctoral fellowship program for supporting this work We are indebted to S L Oscurato

and M Saccone for assistance with photoalignment patterning and DSC experiments

respectively We thank Dr Nelson Tabiryan from BEAM co (httpwwwbeamcocom) for

providing the photoalignment material PAAD-72 for our use

3

pupil size reduces the light transmission by a factor of 7 The light-responsive materials

design together with photoalignment-based control over the molecular orientation provide a

new approach to automatic self-regulating optical systems based on soft smart materials

Depending on biological species pupils come in diverse geometries comprising ellipses slits

pin-holes and crescent shapes helping animals to adapt to the specific requirements of their

living environment[1] In human iris the sphincter and dilator muscles control the pupil size in

order to stabilize the light transmission into the retina[2] In all cases a disturbance in the iris

functionality leads to severe visual dysfunctions Just as in humananimal eyes adjustable

apertures are vital in technological applications particularly in imaging systems and

spectrometers Photographic devices commonly contain a mechanical iris diaphragm

consisting of several thin segments that form an adjustable polygonal aperture which allows

controlling the depth of field optical aberrations and other parameters that influence the

image quality[3] Spectrometers implement tunable slits to control the resolution and to adjust

the overall signal level during data collection[4] Recently compact and tunable aperture

devices have been made using different actuation mechanisms eg electromagnetic

heating[5] electro-wetting in micro-fluidics[67] and piezo-mechanics[8] All these iris-like

devices are passive in the sense that they require external control (ie a separate controller

apart from the iris) and ex-situ light detection schemes An iris device that can self-adjust the

aperture size in response to incident light intensity would provide access to automated optical

systems in environments where no controlling circuits can access eg opto-fluidic channels

However their technical realization remains a grand challenge

The only way to realize an iris device that senses the light field is to fabricate it from a

material that itself is photo-responsive and devise a configuration that openscloses in

response to decreasingincreasing light levels As we show such actuation can be reached in

light-responsive liquid-crystal elastomers (LCEs) which are smart materials that can exhibit

4

large shape deformation upon illumination[9-11] Light-responsive LCEs comprise a loosely

crosslinked network of liquid-crystalline moieties with controlled molecular alignment

whereas the light-active units can be either part of the polymer network[12] or simply doped

into it[13] The elastic network can undergo reversible shape changes in response to

photochemical[9] (eg trans-cis isomerization) or photothermal[14] stimulus In either case the

benefit of the light-based actuation scheme relies on remote non-contact energy supply and

the precise spatial and temporal control over light properties Several LCE-based soft-robotic

devices have been reported exploiting these features such as micro-swimmers[1516]

walkers[1718] and transportation devices[19] Self-regulating behavior in light-driven LCE film

has been demonstrated in simple photomechanical oscillating systems[20] and recently in an

artificial flytrap device capable of autonomous action based on optical feedback[21]

Herein we report an LCE iris that can autonomously open and close by responding to

the incident light intensity without any need for external control circuitry Mimicking the

properties of a natural iris (Figure 1ab) the device openscloses upon decreasingincreasing

light levels To accomplish the first goal we use LCE-based soft actuator and light-induced

bending as the mechanism responsible for the modification of the aperture size To fulfil the

second point we implement photoalignment-based control over molecular alignment to

achieve radial molecular orientation within the LCE film and fabricate a structure with 12

radially aligned segments (Figure 1cd) We employ anisotropic thermal expansion within the

polymerized LCE to maintain the iris in the open state under low-light conditions while at

increased exposures light-triggered molecular reorientation forces the iris to close and thus to

reduce the aperture size To the best of our knowledge this is the first demonstration of

artificial iris that is both powered and controlled with light performing transmission

adjustment relying directly on external light intensity

Bending of a thin strip is the most common way to characterize liquid-crystal polymer

actuation under external stimuli[22-25] From the technological point of view it is often most

5

convenient to fabricate an LCE film in a glass cell with thickness d ranging from a few to a

few tens of m and length l of several millimetres In such films bending upon light

absorption is due to the strain difference ∆ε occurring between the two surfaces with a simple

relationship between the bending angle α and the geometry α ∆εld (l gtgtd) Such bending

film can be extremely sensitive for d = 10 μm and l = 5 mm ∆ε as low as 016 may be

sufficient to induce a bending of 90deg Due to this high sensitivity changes in the bending

angle are affected by many parameters such as environmental variations inhomogeneities in

the material composition or imperfections in the film fabrication process It is widely

accepted that the initial bending existing even when the film is not exposed to light is due to

residual stress in the LC network generated during photopolymerization[2627] The nature of

the residual stress is complex depending delicately on the molecular system used Apart from

uniform shrinkage[28] other reasons for building up the residual stress during polymerization

could be for instance opposite diffusion of monoacrylate and diacrylate monomers during

photopolymerization[29] and cis-to-trans isomerization in the darkness after

photopolymerization[26] However the initial bending phenomenon has rarely been connected

with anisotropic thermal expansion[3031] and to the best of our knowledge it has not been

previously utilized in soft actuator design

Liquid crystal elastomers are anisotropic materials exhibiting not only optical

birefringence but also anisotropic thermal expansion governed by molecular orientation[2332]

This is pertinent when the photopolymerization is performed at elevated temperatures in order

to stabilize the LC phase Figure 2a illustrates an LCE film polymerized at temperature Tp

which is higher than the room temperature TR When a strip is cut out from the film it remains

flat only when the environmental temperature Te is equal to Tp After polymerization however

the actuation experiments are typically carried out at room temperature (in our case TR =

22 degC) tens of degC below the Tp In a splay-aligned film different thermal expansion at

different film surfaces will create the stress and thus induce spontaneous bending of the film

6

When the same film is polymerized at TR (Tp = TR) no stress is preserved inside the material

at TR and no spontaneous bending occurs However when Te is elevated the strip bends to the

opposite direction as illustrated in Figure 2b To demonstrate the utility of the anisotropic

thermal expansion we prepared a series of 20 μm thick splay-aligned LCE strips

polymerized them at different temperatures and monitored the bending angles α (upon no

light irradiation) at different temperatures As shown in Figure 2c an LCE strip polymerized

at 40 degC (solid pink dot) shows gradually increasing bending deformation upon slow cooling

reaching the maximum bending at TR A strip polymerized at 20 degC (red dots) remains flat at

TR In Figure 2d we plot the α vs temperature difference ∆T between polymerization and

room temperature showing that for ∆T gt 0 ie when the polymerization temperature is

higher than the operation temperature maximum initial bending occurs when the

polymerization temperature is 20-30 degC higher than the operation temperature With the

photopolymerizable mixture we employed further increase in the polymerization temperature

to 60 degC (∆T = 38 degC) brings the mixture close to the LC-to-isotropic phase transition

temperature Tc = 64 degC in which case even slight absorption from the polymerizing light can

trigger partial phase transition reduce the order parameter and thus decrease the bending

angle of the LCE strip at TR We note that this initial bending (for ∆T gt 0) is in the direction

opposite to the light-induced bending Therefore we can make use of it in our artificial iris

design (Figure 1cd) making it open in the dark and closed upon light illumination which is

opposite to ldquoconventionalrdquo commonly used light-induced bending in LCE films

To realize the LCE iris it is imperative that all the bending segments actuate

symmetrically towards the same central point requiring the LCE constituents to be radially

aligned Obtaining such alignment using conventional preparation procedures such as rubbing

and mask-exposure[173334] is challenging hence we used photoalignment technique[35-37] to

achieve the desired molecular alignment The sample preparation procedure is illustrated in

Figure 3 The LC cell (20 μm thick) was constructed such that at the upper surface the LC

7

molecules adopted homeotropic alignment while the bottom surface was coated with an

azobenzene-based photoalignment layer (PAAD-72 BEAM Co) allowing us to define the

LC alignment with polarization of the incident light (Figure 3bc) To obtain the radial

molecular alignment we used a radial-polarization converter[38] to transform linearly

polarized light into azimuthally polarized as illustrated in Figure 3d The molecules within

the photoalignment layer orient themselves in the direction perpendicular to the polarization

of the incident light hence azimuthal polarization gives rise to radial photoalignment (Figure

3cd) and consequently radial alignment of the photopolymerizable LC mixture close to the

bottom surface of the cell To make the mixture responsive to visible light (we wanted to

avoid using UV light in order to obtain ldquohuman-friendlyldquo action) we doped it with Disperse

Red 1 (DR1 4 mol-) Upon irradiation with wavelengths in the bluendashgreen region of the

spectrum DR1 is known to undergo continuous trans-cis-trans cycling thereby releasing a

significant amount of heat It is the photothermal heating that we employ in our actuator

design The chemical structures of the compounds used in this work are shown in Figure 3e

The polymerization was carried out at 45 degC in order to maximize the initial bending (Figure

2) After polymerization the material has a Youngrsquos modulus of 25 MPa at room temperature

(Figure S1) and its glass transition temperature is below room temperature as confirmed by

the differential scanning calorimetry curves shown in Figure S2 The cross-polarized optical

micrograph of the LCE film shown in the inset of Figure 3f revealed that the radial

molecular alignment was preserved after polymerization After opening the cell a disc with

140 mm diameter (approximately the size of the human iris) with 12 petal-shape segments

was cut out (Figure 3f) After releasing the stress by annealing at 50 degC the device

spontaneously ldquoopened uprdquo and adapted a flower-like shape at room temperature with a

central hole of about 10 mm in diameter (Figure 3g) In this geometry each ldquopetalrdquo segment

is able to deform in response to the local light intensity as shown in Movie 1 where the LCE

segments move in response to a scanned light beam Upon uniform illumination with 470 nm

8

LED light all iris segments bend inwards and the aperture closes (Figure 3h Movie 2) For

more details on the fabrication process we direct the reader to the Supporting Information

To characterize the performance of the device a 488 nm laser beam with a diameter of

12 mm was projected onto the iris and the transmitted light as a function of the incident light

power was measured (Figure 4a) When the incident power was slowly increased from 20 to

120 mW all segments deformed inwards and gradually reduced the aperture size For laser

powers above 120 mW some segments closed completely resulting in a rapid decrease in the

iris transmission At around 180 mW all the segments bent inwards and the aperture closed

reaching transmission as low as 10 The self-regulating iris is able to adjust the transmitted

light power across a 200 mW range approximately 20 mW of light is transmitted for both 30

mW input (corresponding to 70 transmission) and 200 mW input (10 transmission red

line in Figure 4a) Figure 4b shows the iris response dynamics after the device is illuminated

instantaneously with 270 mWcm-2 intensity and the output power through the iris is recorded

The decreasing curve of the output power comprises several steps each of them

corresponding to one or few iris segments closing Differences in the closure time for

different segments are attributed to fabrication defects (eg slight variation of thickness and

molecular alignment between segments) and high sensitivity of the bending actuation Images

of the iris at different stages of the closing action are shown in the insets of Figure 4b

revealing the separated closed segments and asymmetric shape of the aperture at the

intermediate stages The asymmetric shape of the aperture does not harm the image

processing in photographic application but it may offer novel optical functionalities The

diversity in the material design and the versatility of the photoalignment-based patterning

technique[39] also provide the opportunities to realize artificial irises with different shapes and

photodeformation modes mimicking the diverse iris geometries observed for different animal

species[1] Real-time closing action is shown in Movie 3 where an iris is placed in front of a

LED light source and responds to blocking the light beam Figure 4c plots the closure times

9

for the 12 individual segments under different incident light intensities Upon low-intensity

excitation the closure time varies significantly for the different segments whereas the

variation is much smaller at high light intensities The statistical analysis of the closure times

is shown in Figure S3 of the Supporting Information The complete closure action takes place

in 30 s upon 230 mW cm-2 incident light intensity and in less than 5 s upon 300 mW cm-2

irradiation

We attribute the closing action of the iris mostly to photothermal heating of the LCE

film[14] The heating is demonstrated in Movie 4 showing the light-induced temperature

increase of the LCE film imaged with an infrared camera revealing that the iris becomes flat

(all segments closed) when the maximum temperature within the segments reaches 49 degC

This value is consistent with the polymerization temperature (45 degC) in which case no initial

bending due to the anisotropic thermal expansion is expected (see the pink dots in Figure 2c)

Different from photochemical (isomerization-induced) actuation that is strongly influenced by

optical density[9] the light-induced temperature increase is insensitive to light penetration

depth into the material During photothermal actuation both material surfaces experience

similar temperature increase due to fast heat conduction across the relatively thin film The

temperature-dependent actuation properties allow us to also obtain an iris device with

opposite action ie one that is closed in the dark and opens upon light irradiation This is

demonstrated in Movie 5 showing an iris deforming on a hot plate at 50 degC closing up

without light and opening again upon illumination As an additional benefit the system can be

made sensitive to different wavelengths of light simply by changing the dye inducing the

photothermal effect while sensitivity over broader spectral range can be obtained by doping

the system with several dyes or broadband absorbers We note that the dyes do not have to be

azobenzenes[14] as we believe photoisomerization to play only a minor role in the

performance of the iris However if photochemical actuation was implemented instead of

light-induced heating the iris functionality could be obtained also in liquid environment

10

paving way towards optofluidic applications or actuation in biologically relevant

environments

To conclude we report what we believe to be the first artificial iris that can self-

regulate the aperture size in direct response to the incident light intensity variations We

employ anisotropic thermal expansion of LCEs in order to maintain the iris in the open state

in the dark while radial molecular alignment was used to induce circularly symmetric

actuation of the iris segments thus closing the aperture upon illumination with visible light

The diversity of material design and sophisticated form of actuation paves way towards

adjustable soft optics with light-control capacity and offers alternatives to realize micro-

robotic systems with smart features such as self-recognition stabilization and automatic

manipulation

Experimental Section

Materials The LCE actuator is made by polymerization of a mixture that contains 75 mol

of LC monomer 4-Methoxybenzoic acid 4-(6-acryloyloxyhexyloxy)phenyl ester 20 mol of

LC crosslinker 14-Bis-[4-(3-acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene (both

purchased from Synthon chemicals) 4 mol of light-responsive molecule N-Ethyl-N-(2-

hydroxyethyl)-4-(4-nitrophenylazo)aniline (Disperse Red 1 Sigma Aldrich) and 1 mol of

photo-initiator (22-Dimethoxy-2-phenylacetophenone Sigma Aldrich) All compounds were

used as received For sample preparation details see the Supporting Information

Characterization Absorption spectra were measured with a custom-modified UV-Vis

spectrophotometer (Cary 60 UV-Vis Agilent Technologies) with a polarization controller

Optical images and movies were recorded by using a Canon 5D Mark III camera (f = 100 mm

lens) and thermal images were recorded with an infrared camera (FLIR T420BX close-up

2timeslens 50 μm resolution) Spatially filtered light from a continuous-wave laser (488 nm

Coherent Genesis CX SLM) was expanded to a diameter of 12 mm and controlled with a

11

beam shutter before impinging on the soft iris The light impinged onto the iris always from

the side with bent segments in order to avoid shadowing by the exterior sections of the iris

The power of incident and transmitted light were measured with a microscope-slide power

sensor (Thorlabs S170C) For the iris actuation demonstrations (Movies 1 2 3 5) an LED

(Prior Scientific 470 nm) with tunable output power was used for light actuation For

temperature-dependent bending angle measurements LCE strips were immersed in a water

bath on a heating stage An infrared camera was used to monitor the water temperature and

the images at different water temperatures were captured using the Canon camera in the top

view No sensitivity to humidity was found in the LCE For measuring the mechanical

properties a planar-aligned LCE sample with dimensions 10 times 2 times 005 mm3 was cut along

the nematic director and vertically attached to a force sensor and a linear motor stage The

stressndashstrain curve was measured at room temperature Differential scanning calorimetry

measurement was performed with a Mettler Toledo Star DSC821e instrument using

heatingcooling speeds of 10 degC min-1

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author

Acknowledgements

A P gratefully acknowledges the financial support of the European Research Council

(Starting Grant project PHOTOTUNE Agreement No 679646) O W is thankful to the

graduate school of Tampere University of Technology (TUT) and H Z to the TUT

postdoctoral fellowship program for supporting this work We are indebted to S L Oscurato

and M Saccone for assistance with photoalignment patterning and DSC experiments

respectively We thank Dr Nelson Tabiryan from BEAM co (httpwwwbeamcocom) for

providing the photoalignment material PAAD-72 for our use

4

large shape deformation upon illumination[9-11] Light-responsive LCEs comprise a loosely

crosslinked network of liquid-crystalline moieties with controlled molecular alignment

whereas the light-active units can be either part of the polymer network[12] or simply doped

into it[13] The elastic network can undergo reversible shape changes in response to

photochemical[9] (eg trans-cis isomerization) or photothermal[14] stimulus In either case the

benefit of the light-based actuation scheme relies on remote non-contact energy supply and

the precise spatial and temporal control over light properties Several LCE-based soft-robotic

devices have been reported exploiting these features such as micro-swimmers[1516]

walkers[1718] and transportation devices[19] Self-regulating behavior in light-driven LCE film

has been demonstrated in simple photomechanical oscillating systems[20] and recently in an

artificial flytrap device capable of autonomous action based on optical feedback[21]

Herein we report an LCE iris that can autonomously open and close by responding to

the incident light intensity without any need for external control circuitry Mimicking the

properties of a natural iris (Figure 1ab) the device openscloses upon decreasingincreasing

light levels To accomplish the first goal we use LCE-based soft actuator and light-induced

bending as the mechanism responsible for the modification of the aperture size To fulfil the

second point we implement photoalignment-based control over molecular alignment to

achieve radial molecular orientation within the LCE film and fabricate a structure with 12

radially aligned segments (Figure 1cd) We employ anisotropic thermal expansion within the

polymerized LCE to maintain the iris in the open state under low-light conditions while at

increased exposures light-triggered molecular reorientation forces the iris to close and thus to

reduce the aperture size To the best of our knowledge this is the first demonstration of

artificial iris that is both powered and controlled with light performing transmission

adjustment relying directly on external light intensity

Bending of a thin strip is the most common way to characterize liquid-crystal polymer

actuation under external stimuli[22-25] From the technological point of view it is often most

5

convenient to fabricate an LCE film in a glass cell with thickness d ranging from a few to a

few tens of m and length l of several millimetres In such films bending upon light

absorption is due to the strain difference ∆ε occurring between the two surfaces with a simple

relationship between the bending angle α and the geometry α ∆εld (l gtgtd) Such bending

film can be extremely sensitive for d = 10 μm and l = 5 mm ∆ε as low as 016 may be

sufficient to induce a bending of 90deg Due to this high sensitivity changes in the bending

angle are affected by many parameters such as environmental variations inhomogeneities in

the material composition or imperfections in the film fabrication process It is widely

accepted that the initial bending existing even when the film is not exposed to light is due to

residual stress in the LC network generated during photopolymerization[2627] The nature of

the residual stress is complex depending delicately on the molecular system used Apart from

uniform shrinkage[28] other reasons for building up the residual stress during polymerization

could be for instance opposite diffusion of monoacrylate and diacrylate monomers during

photopolymerization[29] and cis-to-trans isomerization in the darkness after

photopolymerization[26] However the initial bending phenomenon has rarely been connected

with anisotropic thermal expansion[3031] and to the best of our knowledge it has not been

previously utilized in soft actuator design

Liquid crystal elastomers are anisotropic materials exhibiting not only optical

birefringence but also anisotropic thermal expansion governed by molecular orientation[2332]

This is pertinent when the photopolymerization is performed at elevated temperatures in order

to stabilize the LC phase Figure 2a illustrates an LCE film polymerized at temperature Tp

which is higher than the room temperature TR When a strip is cut out from the film it remains

flat only when the environmental temperature Te is equal to Tp After polymerization however

the actuation experiments are typically carried out at room temperature (in our case TR =

22 degC) tens of degC below the Tp In a splay-aligned film different thermal expansion at

different film surfaces will create the stress and thus induce spontaneous bending of the film

6

When the same film is polymerized at TR (Tp = TR) no stress is preserved inside the material

at TR and no spontaneous bending occurs However when Te is elevated the strip bends to the

opposite direction as illustrated in Figure 2b To demonstrate the utility of the anisotropic

thermal expansion we prepared a series of 20 μm thick splay-aligned LCE strips

polymerized them at different temperatures and monitored the bending angles α (upon no

light irradiation) at different temperatures As shown in Figure 2c an LCE strip polymerized

at 40 degC (solid pink dot) shows gradually increasing bending deformation upon slow cooling

reaching the maximum bending at TR A strip polymerized at 20 degC (red dots) remains flat at

TR In Figure 2d we plot the α vs temperature difference ∆T between polymerization and

room temperature showing that for ∆T gt 0 ie when the polymerization temperature is

higher than the operation temperature maximum initial bending occurs when the

polymerization temperature is 20-30 degC higher than the operation temperature With the

photopolymerizable mixture we employed further increase in the polymerization temperature

to 60 degC (∆T = 38 degC) brings the mixture close to the LC-to-isotropic phase transition

temperature Tc = 64 degC in which case even slight absorption from the polymerizing light can

trigger partial phase transition reduce the order parameter and thus decrease the bending

angle of the LCE strip at TR We note that this initial bending (for ∆T gt 0) is in the direction

opposite to the light-induced bending Therefore we can make use of it in our artificial iris

design (Figure 1cd) making it open in the dark and closed upon light illumination which is

opposite to ldquoconventionalrdquo commonly used light-induced bending in LCE films

To realize the LCE iris it is imperative that all the bending segments actuate

symmetrically towards the same central point requiring the LCE constituents to be radially

aligned Obtaining such alignment using conventional preparation procedures such as rubbing

and mask-exposure[173334] is challenging hence we used photoalignment technique[35-37] to

achieve the desired molecular alignment The sample preparation procedure is illustrated in

Figure 3 The LC cell (20 μm thick) was constructed such that at the upper surface the LC

7

molecules adopted homeotropic alignment while the bottom surface was coated with an

azobenzene-based photoalignment layer (PAAD-72 BEAM Co) allowing us to define the

LC alignment with polarization of the incident light (Figure 3bc) To obtain the radial

molecular alignment we used a radial-polarization converter[38] to transform linearly

polarized light into azimuthally polarized as illustrated in Figure 3d The molecules within

the photoalignment layer orient themselves in the direction perpendicular to the polarization

of the incident light hence azimuthal polarization gives rise to radial photoalignment (Figure

3cd) and consequently radial alignment of the photopolymerizable LC mixture close to the

bottom surface of the cell To make the mixture responsive to visible light (we wanted to

avoid using UV light in order to obtain ldquohuman-friendlyldquo action) we doped it with Disperse

Red 1 (DR1 4 mol-) Upon irradiation with wavelengths in the bluendashgreen region of the

spectrum DR1 is known to undergo continuous trans-cis-trans cycling thereby releasing a

significant amount of heat It is the photothermal heating that we employ in our actuator

design The chemical structures of the compounds used in this work are shown in Figure 3e

The polymerization was carried out at 45 degC in order to maximize the initial bending (Figure

2) After polymerization the material has a Youngrsquos modulus of 25 MPa at room temperature

(Figure S1) and its glass transition temperature is below room temperature as confirmed by

the differential scanning calorimetry curves shown in Figure S2 The cross-polarized optical

micrograph of the LCE film shown in the inset of Figure 3f revealed that the radial

molecular alignment was preserved after polymerization After opening the cell a disc with

140 mm diameter (approximately the size of the human iris) with 12 petal-shape segments

was cut out (Figure 3f) After releasing the stress by annealing at 50 degC the device

spontaneously ldquoopened uprdquo and adapted a flower-like shape at room temperature with a

central hole of about 10 mm in diameter (Figure 3g) In this geometry each ldquopetalrdquo segment

is able to deform in response to the local light intensity as shown in Movie 1 where the LCE

segments move in response to a scanned light beam Upon uniform illumination with 470 nm

8

LED light all iris segments bend inwards and the aperture closes (Figure 3h Movie 2) For

more details on the fabrication process we direct the reader to the Supporting Information

To characterize the performance of the device a 488 nm laser beam with a diameter of

12 mm was projected onto the iris and the transmitted light as a function of the incident light

power was measured (Figure 4a) When the incident power was slowly increased from 20 to

120 mW all segments deformed inwards and gradually reduced the aperture size For laser

powers above 120 mW some segments closed completely resulting in a rapid decrease in the

iris transmission At around 180 mW all the segments bent inwards and the aperture closed

reaching transmission as low as 10 The self-regulating iris is able to adjust the transmitted

light power across a 200 mW range approximately 20 mW of light is transmitted for both 30

mW input (corresponding to 70 transmission) and 200 mW input (10 transmission red

line in Figure 4a) Figure 4b shows the iris response dynamics after the device is illuminated

instantaneously with 270 mWcm-2 intensity and the output power through the iris is recorded

The decreasing curve of the output power comprises several steps each of them

corresponding to one or few iris segments closing Differences in the closure time for

different segments are attributed to fabrication defects (eg slight variation of thickness and

molecular alignment between segments) and high sensitivity of the bending actuation Images

of the iris at different stages of the closing action are shown in the insets of Figure 4b

revealing the separated closed segments and asymmetric shape of the aperture at the

intermediate stages The asymmetric shape of the aperture does not harm the image

processing in photographic application but it may offer novel optical functionalities The

diversity in the material design and the versatility of the photoalignment-based patterning

technique[39] also provide the opportunities to realize artificial irises with different shapes and

photodeformation modes mimicking the diverse iris geometries observed for different animal

species[1] Real-time closing action is shown in Movie 3 where an iris is placed in front of a

LED light source and responds to blocking the light beam Figure 4c plots the closure times

9

for the 12 individual segments under different incident light intensities Upon low-intensity

excitation the closure time varies significantly for the different segments whereas the

variation is much smaller at high light intensities The statistical analysis of the closure times

is shown in Figure S3 of the Supporting Information The complete closure action takes place

in 30 s upon 230 mW cm-2 incident light intensity and in less than 5 s upon 300 mW cm-2

irradiation

We attribute the closing action of the iris mostly to photothermal heating of the LCE

film[14] The heating is demonstrated in Movie 4 showing the light-induced temperature

increase of the LCE film imaged with an infrared camera revealing that the iris becomes flat

(all segments closed) when the maximum temperature within the segments reaches 49 degC

This value is consistent with the polymerization temperature (45 degC) in which case no initial

bending due to the anisotropic thermal expansion is expected (see the pink dots in Figure 2c)

Different from photochemical (isomerization-induced) actuation that is strongly influenced by

optical density[9] the light-induced temperature increase is insensitive to light penetration

depth into the material During photothermal actuation both material surfaces experience

similar temperature increase due to fast heat conduction across the relatively thin film The

temperature-dependent actuation properties allow us to also obtain an iris device with

opposite action ie one that is closed in the dark and opens upon light irradiation This is

demonstrated in Movie 5 showing an iris deforming on a hot plate at 50 degC closing up

without light and opening again upon illumination As an additional benefit the system can be

made sensitive to different wavelengths of light simply by changing the dye inducing the

photothermal effect while sensitivity over broader spectral range can be obtained by doping

the system with several dyes or broadband absorbers We note that the dyes do not have to be

azobenzenes[14] as we believe photoisomerization to play only a minor role in the

performance of the iris However if photochemical actuation was implemented instead of

light-induced heating the iris functionality could be obtained also in liquid environment

10

paving way towards optofluidic applications or actuation in biologically relevant

environments

To conclude we report what we believe to be the first artificial iris that can self-

regulate the aperture size in direct response to the incident light intensity variations We

employ anisotropic thermal expansion of LCEs in order to maintain the iris in the open state

in the dark while radial molecular alignment was used to induce circularly symmetric

actuation of the iris segments thus closing the aperture upon illumination with visible light

The diversity of material design and sophisticated form of actuation paves way towards

adjustable soft optics with light-control capacity and offers alternatives to realize micro-

robotic systems with smart features such as self-recognition stabilization and automatic

manipulation

Experimental Section

Materials The LCE actuator is made by polymerization of a mixture that contains 75 mol

of LC monomer 4-Methoxybenzoic acid 4-(6-acryloyloxyhexyloxy)phenyl ester 20 mol of

LC crosslinker 14-Bis-[4-(3-acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene (both

purchased from Synthon chemicals) 4 mol of light-responsive molecule N-Ethyl-N-(2-

hydroxyethyl)-4-(4-nitrophenylazo)aniline (Disperse Red 1 Sigma Aldrich) and 1 mol of

photo-initiator (22-Dimethoxy-2-phenylacetophenone Sigma Aldrich) All compounds were

used as received For sample preparation details see the Supporting Information

Characterization Absorption spectra were measured with a custom-modified UV-Vis

spectrophotometer (Cary 60 UV-Vis Agilent Technologies) with a polarization controller

Optical images and movies were recorded by using a Canon 5D Mark III camera (f = 100 mm

lens) and thermal images were recorded with an infrared camera (FLIR T420BX close-up

2timeslens 50 μm resolution) Spatially filtered light from a continuous-wave laser (488 nm

Coherent Genesis CX SLM) was expanded to a diameter of 12 mm and controlled with a

11

beam shutter before impinging on the soft iris The light impinged onto the iris always from

the side with bent segments in order to avoid shadowing by the exterior sections of the iris

The power of incident and transmitted light were measured with a microscope-slide power

sensor (Thorlabs S170C) For the iris actuation demonstrations (Movies 1 2 3 5) an LED

(Prior Scientific 470 nm) with tunable output power was used for light actuation For

temperature-dependent bending angle measurements LCE strips were immersed in a water

bath on a heating stage An infrared camera was used to monitor the water temperature and

the images at different water temperatures were captured using the Canon camera in the top

view No sensitivity to humidity was found in the LCE For measuring the mechanical

properties a planar-aligned LCE sample with dimensions 10 times 2 times 005 mm3 was cut along

the nematic director and vertically attached to a force sensor and a linear motor stage The

stressndashstrain curve was measured at room temperature Differential scanning calorimetry

measurement was performed with a Mettler Toledo Star DSC821e instrument using

heatingcooling speeds of 10 degC min-1

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author

Acknowledgements

A P gratefully acknowledges the financial support of the European Research Council

(Starting Grant project PHOTOTUNE Agreement No 679646) O W is thankful to the

graduate school of Tampere University of Technology (TUT) and H Z to the TUT

postdoctoral fellowship program for supporting this work We are indebted to S L Oscurato

and M Saccone for assistance with photoalignment patterning and DSC experiments

respectively We thank Dr Nelson Tabiryan from BEAM co (httpwwwbeamcocom) for

providing the photoalignment material PAAD-72 for our use

5

convenient to fabricate an LCE film in a glass cell with thickness d ranging from a few to a

few tens of m and length l of several millimetres In such films bending upon light

absorption is due to the strain difference ∆ε occurring between the two surfaces with a simple

relationship between the bending angle α and the geometry α ∆εld (l gtgtd) Such bending

film can be extremely sensitive for d = 10 μm and l = 5 mm ∆ε as low as 016 may be

sufficient to induce a bending of 90deg Due to this high sensitivity changes in the bending

angle are affected by many parameters such as environmental variations inhomogeneities in

the material composition or imperfections in the film fabrication process It is widely

accepted that the initial bending existing even when the film is not exposed to light is due to

residual stress in the LC network generated during photopolymerization[2627] The nature of

the residual stress is complex depending delicately on the molecular system used Apart from

uniform shrinkage[28] other reasons for building up the residual stress during polymerization

could be for instance opposite diffusion of monoacrylate and diacrylate monomers during

photopolymerization[29] and cis-to-trans isomerization in the darkness after

photopolymerization[26] However the initial bending phenomenon has rarely been connected

with anisotropic thermal expansion[3031] and to the best of our knowledge it has not been

previously utilized in soft actuator design

Liquid crystal elastomers are anisotropic materials exhibiting not only optical

birefringence but also anisotropic thermal expansion governed by molecular orientation[2332]

This is pertinent when the photopolymerization is performed at elevated temperatures in order

to stabilize the LC phase Figure 2a illustrates an LCE film polymerized at temperature Tp

which is higher than the room temperature TR When a strip is cut out from the film it remains

flat only when the environmental temperature Te is equal to Tp After polymerization however

the actuation experiments are typically carried out at room temperature (in our case TR =

22 degC) tens of degC below the Tp In a splay-aligned film different thermal expansion at

different film surfaces will create the stress and thus induce spontaneous bending of the film

6

When the same film is polymerized at TR (Tp = TR) no stress is preserved inside the material

at TR and no spontaneous bending occurs However when Te is elevated the strip bends to the

opposite direction as illustrated in Figure 2b To demonstrate the utility of the anisotropic

thermal expansion we prepared a series of 20 μm thick splay-aligned LCE strips

polymerized them at different temperatures and monitored the bending angles α (upon no

light irradiation) at different temperatures As shown in Figure 2c an LCE strip polymerized

at 40 degC (solid pink dot) shows gradually increasing bending deformation upon slow cooling

reaching the maximum bending at TR A strip polymerized at 20 degC (red dots) remains flat at

TR In Figure 2d we plot the α vs temperature difference ∆T between polymerization and

room temperature showing that for ∆T gt 0 ie when the polymerization temperature is

higher than the operation temperature maximum initial bending occurs when the

polymerization temperature is 20-30 degC higher than the operation temperature With the

photopolymerizable mixture we employed further increase in the polymerization temperature

to 60 degC (∆T = 38 degC) brings the mixture close to the LC-to-isotropic phase transition

temperature Tc = 64 degC in which case even slight absorption from the polymerizing light can

trigger partial phase transition reduce the order parameter and thus decrease the bending

angle of the LCE strip at TR We note that this initial bending (for ∆T gt 0) is in the direction

opposite to the light-induced bending Therefore we can make use of it in our artificial iris

design (Figure 1cd) making it open in the dark and closed upon light illumination which is

opposite to ldquoconventionalrdquo commonly used light-induced bending in LCE films

To realize the LCE iris it is imperative that all the bending segments actuate

symmetrically towards the same central point requiring the LCE constituents to be radially

aligned Obtaining such alignment using conventional preparation procedures such as rubbing

and mask-exposure[173334] is challenging hence we used photoalignment technique[35-37] to

achieve the desired molecular alignment The sample preparation procedure is illustrated in

Figure 3 The LC cell (20 μm thick) was constructed such that at the upper surface the LC

7

molecules adopted homeotropic alignment while the bottom surface was coated with an

azobenzene-based photoalignment layer (PAAD-72 BEAM Co) allowing us to define the

LC alignment with polarization of the incident light (Figure 3bc) To obtain the radial

molecular alignment we used a radial-polarization converter[38] to transform linearly

polarized light into azimuthally polarized as illustrated in Figure 3d The molecules within

the photoalignment layer orient themselves in the direction perpendicular to the polarization

of the incident light hence azimuthal polarization gives rise to radial photoalignment (Figure

3cd) and consequently radial alignment of the photopolymerizable LC mixture close to the

bottom surface of the cell To make the mixture responsive to visible light (we wanted to

avoid using UV light in order to obtain ldquohuman-friendlyldquo action) we doped it with Disperse

Red 1 (DR1 4 mol-) Upon irradiation with wavelengths in the bluendashgreen region of the

spectrum DR1 is known to undergo continuous trans-cis-trans cycling thereby releasing a

significant amount of heat It is the photothermal heating that we employ in our actuator

design The chemical structures of the compounds used in this work are shown in Figure 3e

The polymerization was carried out at 45 degC in order to maximize the initial bending (Figure

2) After polymerization the material has a Youngrsquos modulus of 25 MPa at room temperature

(Figure S1) and its glass transition temperature is below room temperature as confirmed by

the differential scanning calorimetry curves shown in Figure S2 The cross-polarized optical

micrograph of the LCE film shown in the inset of Figure 3f revealed that the radial

molecular alignment was preserved after polymerization After opening the cell a disc with

140 mm diameter (approximately the size of the human iris) with 12 petal-shape segments

was cut out (Figure 3f) After releasing the stress by annealing at 50 degC the device

spontaneously ldquoopened uprdquo and adapted a flower-like shape at room temperature with a

central hole of about 10 mm in diameter (Figure 3g) In this geometry each ldquopetalrdquo segment

is able to deform in response to the local light intensity as shown in Movie 1 where the LCE

segments move in response to a scanned light beam Upon uniform illumination with 470 nm

8

LED light all iris segments bend inwards and the aperture closes (Figure 3h Movie 2) For

more details on the fabrication process we direct the reader to the Supporting Information

To characterize the performance of the device a 488 nm laser beam with a diameter of

12 mm was projected onto the iris and the transmitted light as a function of the incident light

power was measured (Figure 4a) When the incident power was slowly increased from 20 to

120 mW all segments deformed inwards and gradually reduced the aperture size For laser

powers above 120 mW some segments closed completely resulting in a rapid decrease in the

iris transmission At around 180 mW all the segments bent inwards and the aperture closed

reaching transmission as low as 10 The self-regulating iris is able to adjust the transmitted

light power across a 200 mW range approximately 20 mW of light is transmitted for both 30

mW input (corresponding to 70 transmission) and 200 mW input (10 transmission red

line in Figure 4a) Figure 4b shows the iris response dynamics after the device is illuminated

instantaneously with 270 mWcm-2 intensity and the output power through the iris is recorded

The decreasing curve of the output power comprises several steps each of them

corresponding to one or few iris segments closing Differences in the closure time for

different segments are attributed to fabrication defects (eg slight variation of thickness and

molecular alignment between segments) and high sensitivity of the bending actuation Images

of the iris at different stages of the closing action are shown in the insets of Figure 4b

revealing the separated closed segments and asymmetric shape of the aperture at the

intermediate stages The asymmetric shape of the aperture does not harm the image

processing in photographic application but it may offer novel optical functionalities The

diversity in the material design and the versatility of the photoalignment-based patterning

technique[39] also provide the opportunities to realize artificial irises with different shapes and

photodeformation modes mimicking the diverse iris geometries observed for different animal

species[1] Real-time closing action is shown in Movie 3 where an iris is placed in front of a

LED light source and responds to blocking the light beam Figure 4c plots the closure times

9

for the 12 individual segments under different incident light intensities Upon low-intensity

excitation the closure time varies significantly for the different segments whereas the

variation is much smaller at high light intensities The statistical analysis of the closure times

is shown in Figure S3 of the Supporting Information The complete closure action takes place

in 30 s upon 230 mW cm-2 incident light intensity and in less than 5 s upon 300 mW cm-2

irradiation

We attribute the closing action of the iris mostly to photothermal heating of the LCE

film[14] The heating is demonstrated in Movie 4 showing the light-induced temperature

increase of the LCE film imaged with an infrared camera revealing that the iris becomes flat

(all segments closed) when the maximum temperature within the segments reaches 49 degC

This value is consistent with the polymerization temperature (45 degC) in which case no initial

bending due to the anisotropic thermal expansion is expected (see the pink dots in Figure 2c)

Different from photochemical (isomerization-induced) actuation that is strongly influenced by

optical density[9] the light-induced temperature increase is insensitive to light penetration

depth into the material During photothermal actuation both material surfaces experience

similar temperature increase due to fast heat conduction across the relatively thin film The

temperature-dependent actuation properties allow us to also obtain an iris device with

opposite action ie one that is closed in the dark and opens upon light irradiation This is

demonstrated in Movie 5 showing an iris deforming on a hot plate at 50 degC closing up

without light and opening again upon illumination As an additional benefit the system can be

made sensitive to different wavelengths of light simply by changing the dye inducing the

photothermal effect while sensitivity over broader spectral range can be obtained by doping

the system with several dyes or broadband absorbers We note that the dyes do not have to be

azobenzenes[14] as we believe photoisomerization to play only a minor role in the

performance of the iris However if photochemical actuation was implemented instead of

light-induced heating the iris functionality could be obtained also in liquid environment

10

paving way towards optofluidic applications or actuation in biologically relevant

environments

To conclude we report what we believe to be the first artificial iris that can self-

regulate the aperture size in direct response to the incident light intensity variations We

employ anisotropic thermal expansion of LCEs in order to maintain the iris in the open state

in the dark while radial molecular alignment was used to induce circularly symmetric

actuation of the iris segments thus closing the aperture upon illumination with visible light

The diversity of material design and sophisticated form of actuation paves way towards

adjustable soft optics with light-control capacity and offers alternatives to realize micro-

robotic systems with smart features such as self-recognition stabilization and automatic

manipulation

Experimental Section

Materials The LCE actuator is made by polymerization of a mixture that contains 75 mol

of LC monomer 4-Methoxybenzoic acid 4-(6-acryloyloxyhexyloxy)phenyl ester 20 mol of

LC crosslinker 14-Bis-[4-(3-acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene (both

purchased from Synthon chemicals) 4 mol of light-responsive molecule N-Ethyl-N-(2-

hydroxyethyl)-4-(4-nitrophenylazo)aniline (Disperse Red 1 Sigma Aldrich) and 1 mol of

photo-initiator (22-Dimethoxy-2-phenylacetophenone Sigma Aldrich) All compounds were

used as received For sample preparation details see the Supporting Information

Characterization Absorption spectra were measured with a custom-modified UV-Vis

spectrophotometer (Cary 60 UV-Vis Agilent Technologies) with a polarization controller

Optical images and movies were recorded by using a Canon 5D Mark III camera (f = 100 mm

lens) and thermal images were recorded with an infrared camera (FLIR T420BX close-up

2timeslens 50 μm resolution) Spatially filtered light from a continuous-wave laser (488 nm

Coherent Genesis CX SLM) was expanded to a diameter of 12 mm and controlled with a

11

beam shutter before impinging on the soft iris The light impinged onto the iris always from

the side with bent segments in order to avoid shadowing by the exterior sections of the iris

The power of incident and transmitted light were measured with a microscope-slide power

sensor (Thorlabs S170C) For the iris actuation demonstrations (Movies 1 2 3 5) an LED

(Prior Scientific 470 nm) with tunable output power was used for light actuation For

temperature-dependent bending angle measurements LCE strips were immersed in a water

bath on a heating stage An infrared camera was used to monitor the water temperature and

the images at different water temperatures were captured using the Canon camera in the top

view No sensitivity to humidity was found in the LCE For measuring the mechanical

properties a planar-aligned LCE sample with dimensions 10 times 2 times 005 mm3 was cut along

the nematic director and vertically attached to a force sensor and a linear motor stage The

stressndashstrain curve was measured at room temperature Differential scanning calorimetry

measurement was performed with a Mettler Toledo Star DSC821e instrument using

heatingcooling speeds of 10 degC min-1

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author

Acknowledgements

A P gratefully acknowledges the financial support of the European Research Council

(Starting Grant project PHOTOTUNE Agreement No 679646) O W is thankful to the

graduate school of Tampere University of Technology (TUT) and H Z to the TUT

postdoctoral fellowship program for supporting this work We are indebted to S L Oscurato

and M Saccone for assistance with photoalignment patterning and DSC experiments

respectively We thank Dr Nelson Tabiryan from BEAM co (httpwwwbeamcocom) for

providing the photoalignment material PAAD-72 for our use

6

When the same film is polymerized at TR (Tp = TR) no stress is preserved inside the material

at TR and no spontaneous bending occurs However when Te is elevated the strip bends to the

opposite direction as illustrated in Figure 2b To demonstrate the utility of the anisotropic

thermal expansion we prepared a series of 20 μm thick splay-aligned LCE strips

polymerized them at different temperatures and monitored the bending angles α (upon no

light irradiation) at different temperatures As shown in Figure 2c an LCE strip polymerized

at 40 degC (solid pink dot) shows gradually increasing bending deformation upon slow cooling

reaching the maximum bending at TR A strip polymerized at 20 degC (red dots) remains flat at

TR In Figure 2d we plot the α vs temperature difference ∆T between polymerization and

room temperature showing that for ∆T gt 0 ie when the polymerization temperature is

higher than the operation temperature maximum initial bending occurs when the

polymerization temperature is 20-30 degC higher than the operation temperature With the

photopolymerizable mixture we employed further increase in the polymerization temperature

to 60 degC (∆T = 38 degC) brings the mixture close to the LC-to-isotropic phase transition

temperature Tc = 64 degC in which case even slight absorption from the polymerizing light can

trigger partial phase transition reduce the order parameter and thus decrease the bending

angle of the LCE strip at TR We note that this initial bending (for ∆T gt 0) is in the direction

opposite to the light-induced bending Therefore we can make use of it in our artificial iris

design (Figure 1cd) making it open in the dark and closed upon light illumination which is

opposite to ldquoconventionalrdquo commonly used light-induced bending in LCE films

To realize the LCE iris it is imperative that all the bending segments actuate

symmetrically towards the same central point requiring the LCE constituents to be radially

aligned Obtaining such alignment using conventional preparation procedures such as rubbing

and mask-exposure[173334] is challenging hence we used photoalignment technique[35-37] to

achieve the desired molecular alignment The sample preparation procedure is illustrated in

Figure 3 The LC cell (20 μm thick) was constructed such that at the upper surface the LC

7

molecules adopted homeotropic alignment while the bottom surface was coated with an

azobenzene-based photoalignment layer (PAAD-72 BEAM Co) allowing us to define the

LC alignment with polarization of the incident light (Figure 3bc) To obtain the radial

molecular alignment we used a radial-polarization converter[38] to transform linearly

polarized light into azimuthally polarized as illustrated in Figure 3d The molecules within

the photoalignment layer orient themselves in the direction perpendicular to the polarization

of the incident light hence azimuthal polarization gives rise to radial photoalignment (Figure

3cd) and consequently radial alignment of the photopolymerizable LC mixture close to the

bottom surface of the cell To make the mixture responsive to visible light (we wanted to

avoid using UV light in order to obtain ldquohuman-friendlyldquo action) we doped it with Disperse

Red 1 (DR1 4 mol-) Upon irradiation with wavelengths in the bluendashgreen region of the

spectrum DR1 is known to undergo continuous trans-cis-trans cycling thereby releasing a

significant amount of heat It is the photothermal heating that we employ in our actuator

design The chemical structures of the compounds used in this work are shown in Figure 3e

The polymerization was carried out at 45 degC in order to maximize the initial bending (Figure

2) After polymerization the material has a Youngrsquos modulus of 25 MPa at room temperature

(Figure S1) and its glass transition temperature is below room temperature as confirmed by

the differential scanning calorimetry curves shown in Figure S2 The cross-polarized optical

micrograph of the LCE film shown in the inset of Figure 3f revealed that the radial

molecular alignment was preserved after polymerization After opening the cell a disc with

140 mm diameter (approximately the size of the human iris) with 12 petal-shape segments

was cut out (Figure 3f) After releasing the stress by annealing at 50 degC the device

spontaneously ldquoopened uprdquo and adapted a flower-like shape at room temperature with a

central hole of about 10 mm in diameter (Figure 3g) In this geometry each ldquopetalrdquo segment

is able to deform in response to the local light intensity as shown in Movie 1 where the LCE

segments move in response to a scanned light beam Upon uniform illumination with 470 nm

8

LED light all iris segments bend inwards and the aperture closes (Figure 3h Movie 2) For

more details on the fabrication process we direct the reader to the Supporting Information

To characterize the performance of the device a 488 nm laser beam with a diameter of

12 mm was projected onto the iris and the transmitted light as a function of the incident light

power was measured (Figure 4a) When the incident power was slowly increased from 20 to

120 mW all segments deformed inwards and gradually reduced the aperture size For laser

powers above 120 mW some segments closed completely resulting in a rapid decrease in the

iris transmission At around 180 mW all the segments bent inwards and the aperture closed

reaching transmission as low as 10 The self-regulating iris is able to adjust the transmitted

light power across a 200 mW range approximately 20 mW of light is transmitted for both 30

mW input (corresponding to 70 transmission) and 200 mW input (10 transmission red

line in Figure 4a) Figure 4b shows the iris response dynamics after the device is illuminated

instantaneously with 270 mWcm-2 intensity and the output power through the iris is recorded

The decreasing curve of the output power comprises several steps each of them

corresponding to one or few iris segments closing Differences in the closure time for

different segments are attributed to fabrication defects (eg slight variation of thickness and

molecular alignment between segments) and high sensitivity of the bending actuation Images

of the iris at different stages of the closing action are shown in the insets of Figure 4b

revealing the separated closed segments and asymmetric shape of the aperture at the

intermediate stages The asymmetric shape of the aperture does not harm the image

processing in photographic application but it may offer novel optical functionalities The

diversity in the material design and the versatility of the photoalignment-based patterning

technique[39] also provide the opportunities to realize artificial irises with different shapes and

photodeformation modes mimicking the diverse iris geometries observed for different animal

species[1] Real-time closing action is shown in Movie 3 where an iris is placed in front of a

LED light source and responds to blocking the light beam Figure 4c plots the closure times

9

for the 12 individual segments under different incident light intensities Upon low-intensity

excitation the closure time varies significantly for the different segments whereas the

variation is much smaller at high light intensities The statistical analysis of the closure times

is shown in Figure S3 of the Supporting Information The complete closure action takes place

in 30 s upon 230 mW cm-2 incident light intensity and in less than 5 s upon 300 mW cm-2

irradiation

We attribute the closing action of the iris mostly to photothermal heating of the LCE

film[14] The heating is demonstrated in Movie 4 showing the light-induced temperature

increase of the LCE film imaged with an infrared camera revealing that the iris becomes flat

(all segments closed) when the maximum temperature within the segments reaches 49 degC

This value is consistent with the polymerization temperature (45 degC) in which case no initial

bending due to the anisotropic thermal expansion is expected (see the pink dots in Figure 2c)

Different from photochemical (isomerization-induced) actuation that is strongly influenced by

optical density[9] the light-induced temperature increase is insensitive to light penetration

depth into the material During photothermal actuation both material surfaces experience

similar temperature increase due to fast heat conduction across the relatively thin film The

temperature-dependent actuation properties allow us to also obtain an iris device with

opposite action ie one that is closed in the dark and opens upon light irradiation This is

demonstrated in Movie 5 showing an iris deforming on a hot plate at 50 degC closing up

without light and opening again upon illumination As an additional benefit the system can be

made sensitive to different wavelengths of light simply by changing the dye inducing the

photothermal effect while sensitivity over broader spectral range can be obtained by doping

the system with several dyes or broadband absorbers We note that the dyes do not have to be

azobenzenes[14] as we believe photoisomerization to play only a minor role in the

performance of the iris However if photochemical actuation was implemented instead of

light-induced heating the iris functionality could be obtained also in liquid environment

10

paving way towards optofluidic applications or actuation in biologically relevant

environments

To conclude we report what we believe to be the first artificial iris that can self-

regulate the aperture size in direct response to the incident light intensity variations We

employ anisotropic thermal expansion of LCEs in order to maintain the iris in the open state

in the dark while radial molecular alignment was used to induce circularly symmetric

actuation of the iris segments thus closing the aperture upon illumination with visible light

The diversity of material design and sophisticated form of actuation paves way towards

adjustable soft optics with light-control capacity and offers alternatives to realize micro-

robotic systems with smart features such as self-recognition stabilization and automatic

manipulation

Experimental Section

Materials The LCE actuator is made by polymerization of a mixture that contains 75 mol

of LC monomer 4-Methoxybenzoic acid 4-(6-acryloyloxyhexyloxy)phenyl ester 20 mol of

LC crosslinker 14-Bis-[4-(3-acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene (both

purchased from Synthon chemicals) 4 mol of light-responsive molecule N-Ethyl-N-(2-

hydroxyethyl)-4-(4-nitrophenylazo)aniline (Disperse Red 1 Sigma Aldrich) and 1 mol of

photo-initiator (22-Dimethoxy-2-phenylacetophenone Sigma Aldrich) All compounds were

used as received For sample preparation details see the Supporting Information

Characterization Absorption spectra were measured with a custom-modified UV-Vis

spectrophotometer (Cary 60 UV-Vis Agilent Technologies) with a polarization controller

Optical images and movies were recorded by using a Canon 5D Mark III camera (f = 100 mm

lens) and thermal images were recorded with an infrared camera (FLIR T420BX close-up

2timeslens 50 μm resolution) Spatially filtered light from a continuous-wave laser (488 nm

Coherent Genesis CX SLM) was expanded to a diameter of 12 mm and controlled with a

11

beam shutter before impinging on the soft iris The light impinged onto the iris always from

the side with bent segments in order to avoid shadowing by the exterior sections of the iris

The power of incident and transmitted light were measured with a microscope-slide power

sensor (Thorlabs S170C) For the iris actuation demonstrations (Movies 1 2 3 5) an LED

(Prior Scientific 470 nm) with tunable output power was used for light actuation For

temperature-dependent bending angle measurements LCE strips were immersed in a water

bath on a heating stage An infrared camera was used to monitor the water temperature and

the images at different water temperatures were captured using the Canon camera in the top

view No sensitivity to humidity was found in the LCE For measuring the mechanical

properties a planar-aligned LCE sample with dimensions 10 times 2 times 005 mm3 was cut along

the nematic director and vertically attached to a force sensor and a linear motor stage The

stressndashstrain curve was measured at room temperature Differential scanning calorimetry

measurement was performed with a Mettler Toledo Star DSC821e instrument using

heatingcooling speeds of 10 degC min-1

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author

Acknowledgements

A P gratefully acknowledges the financial support of the European Research Council

(Starting Grant project PHOTOTUNE Agreement No 679646) O W is thankful to the

graduate school of Tampere University of Technology (TUT) and H Z to the TUT

postdoctoral fellowship program for supporting this work We are indebted to S L Oscurato

and M Saccone for assistance with photoalignment patterning and DSC experiments

respectively We thank Dr Nelson Tabiryan from BEAM co (httpwwwbeamcocom) for

providing the photoalignment material PAAD-72 for our use

7

molecules adopted homeotropic alignment while the bottom surface was coated with an

azobenzene-based photoalignment layer (PAAD-72 BEAM Co) allowing us to define the

LC alignment with polarization of the incident light (Figure 3bc) To obtain the radial

molecular alignment we used a radial-polarization converter[38] to transform linearly

polarized light into azimuthally polarized as illustrated in Figure 3d The molecules within

the photoalignment layer orient themselves in the direction perpendicular to the polarization

of the incident light hence azimuthal polarization gives rise to radial photoalignment (Figure

3cd) and consequently radial alignment of the photopolymerizable LC mixture close to the

bottom surface of the cell To make the mixture responsive to visible light (we wanted to

avoid using UV light in order to obtain ldquohuman-friendlyldquo action) we doped it with Disperse

Red 1 (DR1 4 mol-) Upon irradiation with wavelengths in the bluendashgreen region of the

spectrum DR1 is known to undergo continuous trans-cis-trans cycling thereby releasing a

significant amount of heat It is the photothermal heating that we employ in our actuator

design The chemical structures of the compounds used in this work are shown in Figure 3e

The polymerization was carried out at 45 degC in order to maximize the initial bending (Figure

2) After polymerization the material has a Youngrsquos modulus of 25 MPa at room temperature

(Figure S1) and its glass transition temperature is below room temperature as confirmed by

the differential scanning calorimetry curves shown in Figure S2 The cross-polarized optical

micrograph of the LCE film shown in the inset of Figure 3f revealed that the radial

molecular alignment was preserved after polymerization After opening the cell a disc with

140 mm diameter (approximately the size of the human iris) with 12 petal-shape segments

was cut out (Figure 3f) After releasing the stress by annealing at 50 degC the device

spontaneously ldquoopened uprdquo and adapted a flower-like shape at room temperature with a

central hole of about 10 mm in diameter (Figure 3g) In this geometry each ldquopetalrdquo segment

is able to deform in response to the local light intensity as shown in Movie 1 where the LCE

segments move in response to a scanned light beam Upon uniform illumination with 470 nm

8

LED light all iris segments bend inwards and the aperture closes (Figure 3h Movie 2) For

more details on the fabrication process we direct the reader to the Supporting Information

To characterize the performance of the device a 488 nm laser beam with a diameter of

12 mm was projected onto the iris and the transmitted light as a function of the incident light

power was measured (Figure 4a) When the incident power was slowly increased from 20 to

120 mW all segments deformed inwards and gradually reduced the aperture size For laser

powers above 120 mW some segments closed completely resulting in a rapid decrease in the

iris transmission At around 180 mW all the segments bent inwards and the aperture closed

reaching transmission as low as 10 The self-regulating iris is able to adjust the transmitted

light power across a 200 mW range approximately 20 mW of light is transmitted for both 30

mW input (corresponding to 70 transmission) and 200 mW input (10 transmission red

line in Figure 4a) Figure 4b shows the iris response dynamics after the device is illuminated

instantaneously with 270 mWcm-2 intensity and the output power through the iris is recorded

The decreasing curve of the output power comprises several steps each of them

corresponding to one or few iris segments closing Differences in the closure time for

different segments are attributed to fabrication defects (eg slight variation of thickness and

molecular alignment between segments) and high sensitivity of the bending actuation Images

of the iris at different stages of the closing action are shown in the insets of Figure 4b

revealing the separated closed segments and asymmetric shape of the aperture at the

intermediate stages The asymmetric shape of the aperture does not harm the image

processing in photographic application but it may offer novel optical functionalities The

diversity in the material design and the versatility of the photoalignment-based patterning

technique[39] also provide the opportunities to realize artificial irises with different shapes and

photodeformation modes mimicking the diverse iris geometries observed for different animal

species[1] Real-time closing action is shown in Movie 3 where an iris is placed in front of a

LED light source and responds to blocking the light beam Figure 4c plots the closure times

9

for the 12 individual segments under different incident light intensities Upon low-intensity

excitation the closure time varies significantly for the different segments whereas the

variation is much smaller at high light intensities The statistical analysis of the closure times

is shown in Figure S3 of the Supporting Information The complete closure action takes place

in 30 s upon 230 mW cm-2 incident light intensity and in less than 5 s upon 300 mW cm-2

irradiation

We attribute the closing action of the iris mostly to photothermal heating of the LCE

film[14] The heating is demonstrated in Movie 4 showing the light-induced temperature

increase of the LCE film imaged with an infrared camera revealing that the iris becomes flat

(all segments closed) when the maximum temperature within the segments reaches 49 degC

This value is consistent with the polymerization temperature (45 degC) in which case no initial

bending due to the anisotropic thermal expansion is expected (see the pink dots in Figure 2c)

Different from photochemical (isomerization-induced) actuation that is strongly influenced by

optical density[9] the light-induced temperature increase is insensitive to light penetration

depth into the material During photothermal actuation both material surfaces experience

similar temperature increase due to fast heat conduction across the relatively thin film The

temperature-dependent actuation properties allow us to also obtain an iris device with

opposite action ie one that is closed in the dark and opens upon light irradiation This is

demonstrated in Movie 5 showing an iris deforming on a hot plate at 50 degC closing up

without light and opening again upon illumination As an additional benefit the system can be

made sensitive to different wavelengths of light simply by changing the dye inducing the

photothermal effect while sensitivity over broader spectral range can be obtained by doping

the system with several dyes or broadband absorbers We note that the dyes do not have to be

azobenzenes[14] as we believe photoisomerization to play only a minor role in the

performance of the iris However if photochemical actuation was implemented instead of

light-induced heating the iris functionality could be obtained also in liquid environment

10

paving way towards optofluidic applications or actuation in biologically relevant

environments

To conclude we report what we believe to be the first artificial iris that can self-

regulate the aperture size in direct response to the incident light intensity variations We

employ anisotropic thermal expansion of LCEs in order to maintain the iris in the open state

in the dark while radial molecular alignment was used to induce circularly symmetric

actuation of the iris segments thus closing the aperture upon illumination with visible light

The diversity of material design and sophisticated form of actuation paves way towards

adjustable soft optics with light-control capacity and offers alternatives to realize micro-

robotic systems with smart features such as self-recognition stabilization and automatic

manipulation

Experimental Section

Materials The LCE actuator is made by polymerization of a mixture that contains 75 mol

of LC monomer 4-Methoxybenzoic acid 4-(6-acryloyloxyhexyloxy)phenyl ester 20 mol of

LC crosslinker 14-Bis-[4-(3-acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene (both

purchased from Synthon chemicals) 4 mol of light-responsive molecule N-Ethyl-N-(2-

hydroxyethyl)-4-(4-nitrophenylazo)aniline (Disperse Red 1 Sigma Aldrich) and 1 mol of

photo-initiator (22-Dimethoxy-2-phenylacetophenone Sigma Aldrich) All compounds were

used as received For sample preparation details see the Supporting Information

Characterization Absorption spectra were measured with a custom-modified UV-Vis

spectrophotometer (Cary 60 UV-Vis Agilent Technologies) with a polarization controller

Optical images and movies were recorded by using a Canon 5D Mark III camera (f = 100 mm

lens) and thermal images were recorded with an infrared camera (FLIR T420BX close-up

2timeslens 50 μm resolution) Spatially filtered light from a continuous-wave laser (488 nm

Coherent Genesis CX SLM) was expanded to a diameter of 12 mm and controlled with a

11

beam shutter before impinging on the soft iris The light impinged onto the iris always from

the side with bent segments in order to avoid shadowing by the exterior sections of the iris

The power of incident and transmitted light were measured with a microscope-slide power

sensor (Thorlabs S170C) For the iris actuation demonstrations (Movies 1 2 3 5) an LED

(Prior Scientific 470 nm) with tunable output power was used for light actuation For

temperature-dependent bending angle measurements LCE strips were immersed in a water

bath on a heating stage An infrared camera was used to monitor the water temperature and

the images at different water temperatures were captured using the Canon camera in the top

view No sensitivity to humidity was found in the LCE For measuring the mechanical

properties a planar-aligned LCE sample with dimensions 10 times 2 times 005 mm3 was cut along

the nematic director and vertically attached to a force sensor and a linear motor stage The

stressndashstrain curve was measured at room temperature Differential scanning calorimetry

measurement was performed with a Mettler Toledo Star DSC821e instrument using

heatingcooling speeds of 10 degC min-1

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author

Acknowledgements

A P gratefully acknowledges the financial support of the European Research Council

(Starting Grant project PHOTOTUNE Agreement No 679646) O W is thankful to the

graduate school of Tampere University of Technology (TUT) and H Z to the TUT

postdoctoral fellowship program for supporting this work We are indebted to S L Oscurato

and M Saccone for assistance with photoalignment patterning and DSC experiments

respectively We thank Dr Nelson Tabiryan from BEAM co (httpwwwbeamcocom) for

providing the photoalignment material PAAD-72 for our use

8

LED light all iris segments bend inwards and the aperture closes (Figure 3h Movie 2) For

more details on the fabrication process we direct the reader to the Supporting Information

To characterize the performance of the device a 488 nm laser beam with a diameter of

12 mm was projected onto the iris and the transmitted light as a function of the incident light

power was measured (Figure 4a) When the incident power was slowly increased from 20 to

120 mW all segments deformed inwards and gradually reduced the aperture size For laser

powers above 120 mW some segments closed completely resulting in a rapid decrease in the

iris transmission At around 180 mW all the segments bent inwards and the aperture closed

reaching transmission as low as 10 The self-regulating iris is able to adjust the transmitted

light power across a 200 mW range approximately 20 mW of light is transmitted for both 30

mW input (corresponding to 70 transmission) and 200 mW input (10 transmission red

line in Figure 4a) Figure 4b shows the iris response dynamics after the device is illuminated

instantaneously with 270 mWcm-2 intensity and the output power through the iris is recorded

The decreasing curve of the output power comprises several steps each of them

corresponding to one or few iris segments closing Differences in the closure time for

different segments are attributed to fabrication defects (eg slight variation of thickness and

molecular alignment between segments) and high sensitivity of the bending actuation Images

of the iris at different stages of the closing action are shown in the insets of Figure 4b

revealing the separated closed segments and asymmetric shape of the aperture at the

intermediate stages The asymmetric shape of the aperture does not harm the image

processing in photographic application but it may offer novel optical functionalities The

diversity in the material design and the versatility of the photoalignment-based patterning

technique[39] also provide the opportunities to realize artificial irises with different shapes and

photodeformation modes mimicking the diverse iris geometries observed for different animal

species[1] Real-time closing action is shown in Movie 3 where an iris is placed in front of a

LED light source and responds to blocking the light beam Figure 4c plots the closure times

9

for the 12 individual segments under different incident light intensities Upon low-intensity

excitation the closure time varies significantly for the different segments whereas the

variation is much smaller at high light intensities The statistical analysis of the closure times

is shown in Figure S3 of the Supporting Information The complete closure action takes place

in 30 s upon 230 mW cm-2 incident light intensity and in less than 5 s upon 300 mW cm-2

irradiation

We attribute the closing action of the iris mostly to photothermal heating of the LCE

film[14] The heating is demonstrated in Movie 4 showing the light-induced temperature

increase of the LCE film imaged with an infrared camera revealing that the iris becomes flat

(all segments closed) when the maximum temperature within the segments reaches 49 degC

This value is consistent with the polymerization temperature (45 degC) in which case no initial

bending due to the anisotropic thermal expansion is expected (see the pink dots in Figure 2c)

Different from photochemical (isomerization-induced) actuation that is strongly influenced by

optical density[9] the light-induced temperature increase is insensitive to light penetration

depth into the material During photothermal actuation both material surfaces experience

similar temperature increase due to fast heat conduction across the relatively thin film The

temperature-dependent actuation properties allow us to also obtain an iris device with

opposite action ie one that is closed in the dark and opens upon light irradiation This is

demonstrated in Movie 5 showing an iris deforming on a hot plate at 50 degC closing up

without light and opening again upon illumination As an additional benefit the system can be

made sensitive to different wavelengths of light simply by changing the dye inducing the

photothermal effect while sensitivity over broader spectral range can be obtained by doping

the system with several dyes or broadband absorbers We note that the dyes do not have to be

azobenzenes[14] as we believe photoisomerization to play only a minor role in the

performance of the iris However if photochemical actuation was implemented instead of

light-induced heating the iris functionality could be obtained also in liquid environment

10

paving way towards optofluidic applications or actuation in biologically relevant

environments

To conclude we report what we believe to be the first artificial iris that can self-

regulate the aperture size in direct response to the incident light intensity variations We

employ anisotropic thermal expansion of LCEs in order to maintain the iris in the open state

in the dark while radial molecular alignment was used to induce circularly symmetric

actuation of the iris segments thus closing the aperture upon illumination with visible light

The diversity of material design and sophisticated form of actuation paves way towards

adjustable soft optics with light-control capacity and offers alternatives to realize micro-

robotic systems with smart features such as self-recognition stabilization and automatic

manipulation

Experimental Section

Materials The LCE actuator is made by polymerization of a mixture that contains 75 mol

of LC monomer 4-Methoxybenzoic acid 4-(6-acryloyloxyhexyloxy)phenyl ester 20 mol of

LC crosslinker 14-Bis-[4-(3-acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene (both

purchased from Synthon chemicals) 4 mol of light-responsive molecule N-Ethyl-N-(2-

hydroxyethyl)-4-(4-nitrophenylazo)aniline (Disperse Red 1 Sigma Aldrich) and 1 mol of

photo-initiator (22-Dimethoxy-2-phenylacetophenone Sigma Aldrich) All compounds were

used as received For sample preparation details see the Supporting Information

Characterization Absorption spectra were measured with a custom-modified UV-Vis

spectrophotometer (Cary 60 UV-Vis Agilent Technologies) with a polarization controller

Optical images and movies were recorded by using a Canon 5D Mark III camera (f = 100 mm

lens) and thermal images were recorded with an infrared camera (FLIR T420BX close-up

2timeslens 50 μm resolution) Spatially filtered light from a continuous-wave laser (488 nm

Coherent Genesis CX SLM) was expanded to a diameter of 12 mm and controlled with a

11

beam shutter before impinging on the soft iris The light impinged onto the iris always from

the side with bent segments in order to avoid shadowing by the exterior sections of the iris

The power of incident and transmitted light were measured with a microscope-slide power

sensor (Thorlabs S170C) For the iris actuation demonstrations (Movies 1 2 3 5) an LED

(Prior Scientific 470 nm) with tunable output power was used for light actuation For

temperature-dependent bending angle measurements LCE strips were immersed in a water

bath on a heating stage An infrared camera was used to monitor the water temperature and

the images at different water temperatures were captured using the Canon camera in the top

view No sensitivity to humidity was found in the LCE For measuring the mechanical

properties a planar-aligned LCE sample with dimensions 10 times 2 times 005 mm3 was cut along

the nematic director and vertically attached to a force sensor and a linear motor stage The

stressndashstrain curve was measured at room temperature Differential scanning calorimetry

measurement was performed with a Mettler Toledo Star DSC821e instrument using

heatingcooling speeds of 10 degC min-1

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author

Acknowledgements

A P gratefully acknowledges the financial support of the European Research Council

(Starting Grant project PHOTOTUNE Agreement No 679646) O W is thankful to the

graduate school of Tampere University of Technology (TUT) and H Z to the TUT

postdoctoral fellowship program for supporting this work We are indebted to S L Oscurato

and M Saccone for assistance with photoalignment patterning and DSC experiments

respectively We thank Dr Nelson Tabiryan from BEAM co (httpwwwbeamcocom) for

providing the photoalignment material PAAD-72 for our use

9

for the 12 individual segments under different incident light intensities Upon low-intensity

excitation the closure time varies significantly for the different segments whereas the

variation is much smaller at high light intensities The statistical analysis of the closure times

is shown in Figure S3 of the Supporting Information The complete closure action takes place

in 30 s upon 230 mW cm-2 incident light intensity and in less than 5 s upon 300 mW cm-2

irradiation

We attribute the closing action of the iris mostly to photothermal heating of the LCE

film[14] The heating is demonstrated in Movie 4 showing the light-induced temperature

increase of the LCE film imaged with an infrared camera revealing that the iris becomes flat

(all segments closed) when the maximum temperature within the segments reaches 49 degC

This value is consistent with the polymerization temperature (45 degC) in which case no initial

bending due to the anisotropic thermal expansion is expected (see the pink dots in Figure 2c)

Different from photochemical (isomerization-induced) actuation that is strongly influenced by

optical density[9] the light-induced temperature increase is insensitive to light penetration

depth into the material During photothermal actuation both material surfaces experience

similar temperature increase due to fast heat conduction across the relatively thin film The

temperature-dependent actuation properties allow us to also obtain an iris device with

opposite action ie one that is closed in the dark and opens upon light irradiation This is

demonstrated in Movie 5 showing an iris deforming on a hot plate at 50 degC closing up

without light and opening again upon illumination As an additional benefit the system can be

made sensitive to different wavelengths of light simply by changing the dye inducing the

photothermal effect while sensitivity over broader spectral range can be obtained by doping

the system with several dyes or broadband absorbers We note that the dyes do not have to be

azobenzenes[14] as we believe photoisomerization to play only a minor role in the

performance of the iris However if photochemical actuation was implemented instead of

light-induced heating the iris functionality could be obtained also in liquid environment

10

paving way towards optofluidic applications or actuation in biologically relevant

environments

To conclude we report what we believe to be the first artificial iris that can self-

regulate the aperture size in direct response to the incident light intensity variations We

employ anisotropic thermal expansion of LCEs in order to maintain the iris in the open state

in the dark while radial molecular alignment was used to induce circularly symmetric

actuation of the iris segments thus closing the aperture upon illumination with visible light

The diversity of material design and sophisticated form of actuation paves way towards

adjustable soft optics with light-control capacity and offers alternatives to realize micro-

robotic systems with smart features such as self-recognition stabilization and automatic

manipulation

Experimental Section

Materials The LCE actuator is made by polymerization of a mixture that contains 75 mol

of LC monomer 4-Methoxybenzoic acid 4-(6-acryloyloxyhexyloxy)phenyl ester 20 mol of

LC crosslinker 14-Bis-[4-(3-acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene (both

purchased from Synthon chemicals) 4 mol of light-responsive molecule N-Ethyl-N-(2-

hydroxyethyl)-4-(4-nitrophenylazo)aniline (Disperse Red 1 Sigma Aldrich) and 1 mol of

photo-initiator (22-Dimethoxy-2-phenylacetophenone Sigma Aldrich) All compounds were

used as received For sample preparation details see the Supporting Information

Characterization Absorption spectra were measured with a custom-modified UV-Vis

spectrophotometer (Cary 60 UV-Vis Agilent Technologies) with a polarization controller

Optical images and movies were recorded by using a Canon 5D Mark III camera (f = 100 mm

lens) and thermal images were recorded with an infrared camera (FLIR T420BX close-up

2timeslens 50 μm resolution) Spatially filtered light from a continuous-wave laser (488 nm

Coherent Genesis CX SLM) was expanded to a diameter of 12 mm and controlled with a

11

beam shutter before impinging on the soft iris The light impinged onto the iris always from

the side with bent segments in order to avoid shadowing by the exterior sections of the iris

The power of incident and transmitted light were measured with a microscope-slide power

sensor (Thorlabs S170C) For the iris actuation demonstrations (Movies 1 2 3 5) an LED

(Prior Scientific 470 nm) with tunable output power was used for light actuation For

temperature-dependent bending angle measurements LCE strips were immersed in a water

bath on a heating stage An infrared camera was used to monitor the water temperature and

the images at different water temperatures were captured using the Canon camera in the top

view No sensitivity to humidity was found in the LCE For measuring the mechanical

properties a planar-aligned LCE sample with dimensions 10 times 2 times 005 mm3 was cut along

the nematic director and vertically attached to a force sensor and a linear motor stage The

stressndashstrain curve was measured at room temperature Differential scanning calorimetry

measurement was performed with a Mettler Toledo Star DSC821e instrument using

heatingcooling speeds of 10 degC min-1

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author

Acknowledgements

A P gratefully acknowledges the financial support of the European Research Council

(Starting Grant project PHOTOTUNE Agreement No 679646) O W is thankful to the

graduate school of Tampere University of Technology (TUT) and H Z to the TUT

postdoctoral fellowship program for supporting this work We are indebted to S L Oscurato

and M Saccone for assistance with photoalignment patterning and DSC experiments

respectively We thank Dr Nelson Tabiryan from BEAM co (httpwwwbeamcocom) for

providing the photoalignment material PAAD-72 for our use

10

paving way towards optofluidic applications or actuation in biologically relevant

environments

To conclude we report what we believe to be the first artificial iris that can self-

regulate the aperture size in direct response to the incident light intensity variations We

employ anisotropic thermal expansion of LCEs in order to maintain the iris in the open state

in the dark while radial molecular alignment was used to induce circularly symmetric

actuation of the iris segments thus closing the aperture upon illumination with visible light

The diversity of material design and sophisticated form of actuation paves way towards

adjustable soft optics with light-control capacity and offers alternatives to realize micro-

robotic systems with smart features such as self-recognition stabilization and automatic

manipulation

Experimental Section

Materials The LCE actuator is made by polymerization of a mixture that contains 75 mol

of LC monomer 4-Methoxybenzoic acid 4-(6-acryloyloxyhexyloxy)phenyl ester 20 mol of

LC crosslinker 14-Bis-[4-(3-acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene (both

purchased from Synthon chemicals) 4 mol of light-responsive molecule N-Ethyl-N-(2-

hydroxyethyl)-4-(4-nitrophenylazo)aniline (Disperse Red 1 Sigma Aldrich) and 1 mol of

photo-initiator (22-Dimethoxy-2-phenylacetophenone Sigma Aldrich) All compounds were

used as received For sample preparation details see the Supporting Information

Characterization Absorption spectra were measured with a custom-modified UV-Vis

spectrophotometer (Cary 60 UV-Vis Agilent Technologies) with a polarization controller

Optical images and movies were recorded by using a Canon 5D Mark III camera (f = 100 mm

lens) and thermal images were recorded with an infrared camera (FLIR T420BX close-up

2timeslens 50 μm resolution) Spatially filtered light from a continuous-wave laser (488 nm

Coherent Genesis CX SLM) was expanded to a diameter of 12 mm and controlled with a

11

beam shutter before impinging on the soft iris The light impinged onto the iris always from

the side with bent segments in order to avoid shadowing by the exterior sections of the iris

The power of incident and transmitted light were measured with a microscope-slide power

sensor (Thorlabs S170C) For the iris actuation demonstrations (Movies 1 2 3 5) an LED

(Prior Scientific 470 nm) with tunable output power was used for light actuation For

temperature-dependent bending angle measurements LCE strips were immersed in a water

bath on a heating stage An infrared camera was used to monitor the water temperature and

the images at different water temperatures were captured using the Canon camera in the top

view No sensitivity to humidity was found in the LCE For measuring the mechanical

properties a planar-aligned LCE sample with dimensions 10 times 2 times 005 mm3 was cut along

the nematic director and vertically attached to a force sensor and a linear motor stage The

stressndashstrain curve was measured at room temperature Differential scanning calorimetry

measurement was performed with a Mettler Toledo Star DSC821e instrument using

heatingcooling speeds of 10 degC min-1

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author

Acknowledgements

A P gratefully acknowledges the financial support of the European Research Council

(Starting Grant project PHOTOTUNE Agreement No 679646) O W is thankful to the

graduate school of Tampere University of Technology (TUT) and H Z to the TUT

postdoctoral fellowship program for supporting this work We are indebted to S L Oscurato

and M Saccone for assistance with photoalignment patterning and DSC experiments

respectively We thank Dr Nelson Tabiryan from BEAM co (httpwwwbeamcocom) for

providing the photoalignment material PAAD-72 for our use

11

beam shutter before impinging on the soft iris The light impinged onto the iris always from

the side with bent segments in order to avoid shadowing by the exterior sections of the iris

The power of incident and transmitted light were measured with a microscope-slide power

sensor (Thorlabs S170C) For the iris actuation demonstrations (Movies 1 2 3 5) an LED

(Prior Scientific 470 nm) with tunable output power was used for light actuation For

temperature-dependent bending angle measurements LCE strips were immersed in a water

bath on a heating stage An infrared camera was used to monitor the water temperature and

the images at different water temperatures were captured using the Canon camera in the top

view No sensitivity to humidity was found in the LCE For measuring the mechanical

properties a planar-aligned LCE sample with dimensions 10 times 2 times 005 mm3 was cut along

the nematic director and vertically attached to a force sensor and a linear motor stage The

stressndashstrain curve was measured at room temperature Differential scanning calorimetry

measurement was performed with a Mettler Toledo Star DSC821e instrument using

heatingcooling speeds of 10 degC min-1

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author

Acknowledgements

A P gratefully acknowledges the financial support of the European Research Council

(Starting Grant project PHOTOTUNE Agreement No 679646) O W is thankful to the

graduate school of Tampere University of Technology (TUT) and H Z to the TUT

postdoctoral fellowship program for supporting this work We are indebted to S L Oscurato

and M Saccone for assistance with photoalignment patterning and DSC experiments

respectively We thank Dr Nelson Tabiryan from BEAM co (httpwwwbeamcocom) for

providing the photoalignment material PAAD-72 for our use

12

Received ((will be filled in by the editorial staff))

Revised ((will be filled in by the editorial staff))

Published online ((will be filled in by the editorial staff))

References

1 J Gonzaacutelez-Martiacuten-Moro F Goacutemez-Sanz A Sales-Sanz E Huguet-Baudin J Murube-

del-Castillo Arch Soc Esp Oftalmol 2014 89 484

2 R S Snell and M A Lemp Movements of the eyeball and the extraocular muscles in

Clinical Anatomy of the Eye Second Edition Blackwell Science Ltd Oxford UK 1997

3 E Allen and S Triantaphillidou The Manual of Photography (10th Edition) Elsevier Ltd

Oxford UK 2012

4 J M Lerner Cytom Part A 2006 69A 712

5 S Schuhladen F Preller R Rix S Petsch R Zentel H Zappe Adv Mater 2014 26

7247

6 P Muller R Feuerstein H Zappe J Microelectromech Syst 2012 21 1156

7 S Schuhladen K Banerjee M Sturmer P Muller U Wallrabe H Zappe Light Sci Appl

2016 5 e16005

8 J Draheim T Burger J G Korvink U Wallrabe Opt Lett 2011 36 2032

9 T Ikeda J-i Mamiya Y Yu Angew Chem Int Ed 2007 46 506

10 C Ohm M Brehmer R Zentel Adv Mater 2010 22 3366

11 M H Li P Keller B Li X Wang M Brunet Adv Mater 2003 15 569

12 C L van Oosten C W M Bastiaansen D J Broer Nat Mater 2009 8 677

13 M Camacho-Lopez H Finkelmann P Palffy-Muhoray M Shelley Nat Mater 2004 3

307

14 A H Gelebart G Vantomme B E W Meijer D J Broer Adv Mater DOI

101002adma201606712

15 C Huang J-a Lv X Tian Y Wang Y Yu J Liu Sci Rep 2015 5 17414

13

16 S Palagi A G Mark S Y Reigh K Melde T Qiu H Zeng C Parmeggiani D

Martella A Sanchez-Castillo N Kapernaum F Giesselmann D S Wiersma E Lauga P

Fischer Nat Mater 2016 15 647

17 M Rogoz H Zeng C Xuan D S Wiersma P Wasylczyk Adv Opt Mater 2016 4

1689

18 H Zeng P Wasylczyk C Parmeggiani D Martella M Burresi D S Wiersma Adv

Mater 2015 27 3883

19 E Kizilkan J Strueben A Staubitz S N Gorb Sci Robot 2017 2 eaak9454

20 T J White N V Tabiryan S V Serak U A Hrozhyk V P Tondiglia H Koerner R

A Vaiaa T J Bunning Soft Matter 2008 4 1796

21 O M Wani H Zeng A Priimagi Nat Commun in press

22 S Iamsaard S J Aszlighoff B Matt T Kudernac J J L M Cornelissen S P Fletcher N

Katsonis Nat Chem 2014 6 229

23 GthinspN Mol KthinspD Harris CthinspWthinspM Bastiaansen DthinspJ Broer Adv Funct Mater 2005 15

1155

24 D Corbett M Warner Liq Cryst 2009 36 1263

25 C L van Oosten D Corbett D Davies M Warner C W M Bastiaansen D J Broer

Macromolecules 2008 41 8592

26 K Kumar C Knie D Bleger M A Peletier H Friedrich S Hecht D J Broer M G

Debije A P H J Schenning Nat Commun 2016 7 11975

27 Y Sawa F Ye K Urayama T Takigawa V Gimenez-Pinto R L B Selinger J V

Selinger Proc Natl Acad Sci USA 2011 108 6364

28 R M Carvalho J C Pereira M Yoshiyama D H Pashley Oper Dent 1996 21 17

29 C F van Nostrum R J M Nolte D J Broer T Fuhrman J H Wendorff Chem Mater

1998 10 135

14

30 L T de Haan V Gimenez-Pinto A Konya T-S Nguyen J M N Verjans C Saacutenchez-

Somolinos J V Selinger R L B Selinger D J Broer Adv Funct Mater 2014 24 1251

31 J J Wie K M Lee T H Ware T J White Macromolecules 2015 48 1087

32 D J Broer G N Mol Polym Eng Sci 1991 31 625

33 S Varghese S Narayanankutty C W M Bastiaansen G P Crawford D J Broer Adv

Matt 2004 16 1600

34 L T de Haan C Saacutenchez-Somolinos C M W Bastiaansen A P H J Schenning D J

Broer Angew Chem Int Ed 2012 51 12469

35 T H Ware M E McConney J J Wie V P Tondiglia T J White Science 2015 347

982

36 O Yaroshchuk Y Reznikov J Mater Chem 2011 22 286

37 T Seki S Nagano M Hara Polymer 2013 54 6053

38 M Stalder M Schadt Opt Lett 1996 21 1948

39 M E McConney A Martinez V P Tondiglia K M Lee D Langley I I Smalyukh T

J White Adv Mater 2013 25 5880

15

Figure 1 a Human iris closes in a bright environment and b opens in the dark c Schematic

drawing of the light-driven iris at its closed state where light illumination triggers petal

segments to bend inwards and to reduce the aperture size d The iris is open under weakno

light conditions when the anisotropic thermal expansion induces bending in the segments

thus opening the aperture

16

Figure 2 a Schematic drawing of a splay-bend LCE strip polymerized at an elevated

temperature (Tp gt TR) It remains flat when the environmental temperature Te = Tp and

spontaneously bends towards the homeotropic direction due to anisotropic thermal expansion

when the temperature is reduced to TR b The same strip polymerized at TR remains flat at TR

and bends to the opposite direction at an elevated temperature (Te gt TR) c The measured

bending angle α of LCE strips polymerized at different temperatures Tp The two LCE strips

remain flat at the polymerization temperature (α = 0 solid dots on the x-axis) and bend with

different angles when environmental temperature is changed (empty circles) Insets

photographs of the LCE strips (6times1times002 mm3) at different temperatures d Bending angle vs

temperature difference T between polymerization temperature and room temperature Inset

schematic drawing of the anisotropic thermal expansion bending direction with respect to the

splayed molecular orientation within the strip Error bars in c and b indicate standard

deviation for 3 independent measurements

17

Figure 3 Schematic of the soft iris fabrication process a A 20 microm cell is prepared by

placing together two glass slides coated with a homeotropic alignment layer on the top and

photoalignment layer at the bottom b LC monomer mixture is infiltrated into the cell and it

adapts a centrally-symmetric splayed alignment directed by c molecular orientation at the

top and bottom interfaces d Schematic of the photoalignment setup A linearly polarized 488

nm laser beam is converted into azimuthally polarized by using a polarization converter and

then projected onto the substrate coated with the photoalignment layer due to which the

azobenzene molecules within the photoalignment layer adapt radial distribution e Chemical

composition of the LC mixture used f Optical image of the polymerized LCE film cut out

with a circle and with 12 petal-like segments Inset cross-polarized image of the film before

cutting g Soft LCE iris in the open state when no light impinges on it h The iris closes upon

light illumination (470 nm 250 mW cm-2) Scale bars are 5 mm

18

Figure 4 a Measured transmission (black dots) and transmitted power (red dots) at different

input powers Light source 488 nm laser with circular polarization Beam diameter 12 mm

Error bars indicate standard deviation for 3 independent measurements Transmitted power

presents the average output power from 3 independent measurements and the dashed line

represents the case of 100 transmission through the device b Measured output power at

different times after switching on the light at t = 0 s Intensity 270 mWcm-2 Insets

photographs of the iris at different stages An optical filter is used to block wavelengths below

500 nm c Closure time for 12 petal segments at different illumination intensities The last

closed segment defines the complete closure time

19

The table of contents

Iris is a biological tissue that can change the pupil size to stabilize the light transmission into

the eye We report an iris-inspired soft device that can automatically adjust aperture size in

response to the change of incident light intensity The device is made of radially photoaligned

liquid crystal elastomer using anisotropic thermal expansion to realize the desired

photoactuation mode

Keywords iris biomimetic photoactuation liquid crystal elastomer azobenzene

Hao Zeng Owies M Wani Piotr Wasylczyk Radosław Kaczmarek Arri Priimagi

Self-regulating iris based on light-actuable liquid crystal elastomer

ToC figure

13

16 S Palagi A G Mark S Y Reigh K Melde T Qiu H Zeng C Parmeggiani D

Martella A Sanchez-Castillo N Kapernaum F Giesselmann D S Wiersma E Lauga P

Fischer Nat Mater 2016 15 647

17 M Rogoz H Zeng C Xuan D S Wiersma P Wasylczyk Adv Opt Mater 2016 4

1689

18 H Zeng P Wasylczyk C Parmeggiani D Martella M Burresi D S Wiersma Adv

Mater 2015 27 3883

19 E Kizilkan J Strueben A Staubitz S N Gorb Sci Robot 2017 2 eaak9454

20 T J White N V Tabiryan S V Serak U A Hrozhyk V P Tondiglia H Koerner R

A Vaiaa T J Bunning Soft Matter 2008 4 1796

21 O M Wani H Zeng A Priimagi Nat Commun in press

22 S Iamsaard S J Aszlighoff B Matt T Kudernac J J L M Cornelissen S P Fletcher N

Katsonis Nat Chem 2014 6 229

23 GthinspN Mol KthinspD Harris CthinspWthinspM Bastiaansen DthinspJ Broer Adv Funct Mater 2005 15

1155

24 D Corbett M Warner Liq Cryst 2009 36 1263

25 C L van Oosten D Corbett D Davies M Warner C W M Bastiaansen D J Broer

Macromolecules 2008 41 8592

26 K Kumar C Knie D Bleger M A Peletier H Friedrich S Hecht D J Broer M G

Debije A P H J Schenning Nat Commun 2016 7 11975

27 Y Sawa F Ye K Urayama T Takigawa V Gimenez-Pinto R L B Selinger J V

Selinger Proc Natl Acad Sci USA 2011 108 6364

28 R M Carvalho J C Pereira M Yoshiyama D H Pashley Oper Dent 1996 21 17

29 C F van Nostrum R J M Nolte D J Broer T Fuhrman J H Wendorff Chem Mater

1998 10 135

14

30 L T de Haan V Gimenez-Pinto A Konya T-S Nguyen J M N Verjans C Saacutenchez-

Somolinos J V Selinger R L B Selinger D J Broer Adv Funct Mater 2014 24 1251

31 J J Wie K M Lee T H Ware T J White Macromolecules 2015 48 1087

32 D J Broer G N Mol Polym Eng Sci 1991 31 625

33 S Varghese S Narayanankutty C W M Bastiaansen G P Crawford D J Broer Adv

Matt 2004 16 1600

34 L T de Haan C Saacutenchez-Somolinos C M W Bastiaansen A P H J Schenning D J

Broer Angew Chem Int Ed 2012 51 12469

35 T H Ware M E McConney J J Wie V P Tondiglia T J White Science 2015 347

982

36 O Yaroshchuk Y Reznikov J Mater Chem 2011 22 286

37 T Seki S Nagano M Hara Polymer 2013 54 6053

38 M Stalder M Schadt Opt Lett 1996 21 1948

39 M E McConney A Martinez V P Tondiglia K M Lee D Langley I I Smalyukh T

J White Adv Mater 2013 25 5880

15

Figure 1 a Human iris closes in a bright environment and b opens in the dark c Schematic

drawing of the light-driven iris at its closed state where light illumination triggers petal

segments to bend inwards and to reduce the aperture size d The iris is open under weakno

light conditions when the anisotropic thermal expansion induces bending in the segments

thus opening the aperture

16

Figure 2 a Schematic drawing of a splay-bend LCE strip polymerized at an elevated

temperature (Tp gt TR) It remains flat when the environmental temperature Te = Tp and

spontaneously bends towards the homeotropic direction due to anisotropic thermal expansion

when the temperature is reduced to TR b The same strip polymerized at TR remains flat at TR

and bends to the opposite direction at an elevated temperature (Te gt TR) c The measured

bending angle α of LCE strips polymerized at different temperatures Tp The two LCE strips

remain flat at the polymerization temperature (α = 0 solid dots on the x-axis) and bend with

different angles when environmental temperature is changed (empty circles) Insets

photographs of the LCE strips (6times1times002 mm3) at different temperatures d Bending angle vs

temperature difference T between polymerization temperature and room temperature Inset

schematic drawing of the anisotropic thermal expansion bending direction with respect to the

splayed molecular orientation within the strip Error bars in c and b indicate standard

deviation for 3 independent measurements

17

Figure 3 Schematic of the soft iris fabrication process a A 20 microm cell is prepared by

placing together two glass slides coated with a homeotropic alignment layer on the top and

photoalignment layer at the bottom b LC monomer mixture is infiltrated into the cell and it

adapts a centrally-symmetric splayed alignment directed by c molecular orientation at the

top and bottom interfaces d Schematic of the photoalignment setup A linearly polarized 488

nm laser beam is converted into azimuthally polarized by using a polarization converter and

then projected onto the substrate coated with the photoalignment layer due to which the

azobenzene molecules within the photoalignment layer adapt radial distribution e Chemical

composition of the LC mixture used f Optical image of the polymerized LCE film cut out

with a circle and with 12 petal-like segments Inset cross-polarized image of the film before

cutting g Soft LCE iris in the open state when no light impinges on it h The iris closes upon

light illumination (470 nm 250 mW cm-2) Scale bars are 5 mm

18

Figure 4 a Measured transmission (black dots) and transmitted power (red dots) at different

input powers Light source 488 nm laser with circular polarization Beam diameter 12 mm

Error bars indicate standard deviation for 3 independent measurements Transmitted power

presents the average output power from 3 independent measurements and the dashed line

represents the case of 100 transmission through the device b Measured output power at

different times after switching on the light at t = 0 s Intensity 270 mWcm-2 Insets

photographs of the iris at different stages An optical filter is used to block wavelengths below

500 nm c Closure time for 12 petal segments at different illumination intensities The last

closed segment defines the complete closure time

19

The table of contents

Iris is a biological tissue that can change the pupil size to stabilize the light transmission into

the eye We report an iris-inspired soft device that can automatically adjust aperture size in

response to the change of incident light intensity The device is made of radially photoaligned

liquid crystal elastomer using anisotropic thermal expansion to realize the desired

photoactuation mode

Keywords iris biomimetic photoactuation liquid crystal elastomer azobenzene

Hao Zeng Owies M Wani Piotr Wasylczyk Radosław Kaczmarek Arri Priimagi

Self-regulating iris based on light-actuable liquid crystal elastomer

ToC figure

14

30 L T de Haan V Gimenez-Pinto A Konya T-S Nguyen J M N Verjans C Saacutenchez-

Somolinos J V Selinger R L B Selinger D J Broer Adv Funct Mater 2014 24 1251

31 J J Wie K M Lee T H Ware T J White Macromolecules 2015 48 1087

32 D J Broer G N Mol Polym Eng Sci 1991 31 625

33 S Varghese S Narayanankutty C W M Bastiaansen G P Crawford D J Broer Adv

Matt 2004 16 1600

34 L T de Haan C Saacutenchez-Somolinos C M W Bastiaansen A P H J Schenning D J

Broer Angew Chem Int Ed 2012 51 12469

35 T H Ware M E McConney J J Wie V P Tondiglia T J White Science 2015 347

982

36 O Yaroshchuk Y Reznikov J Mater Chem 2011 22 286

37 T Seki S Nagano M Hara Polymer 2013 54 6053

38 M Stalder M Schadt Opt Lett 1996 21 1948

39 M E McConney A Martinez V P Tondiglia K M Lee D Langley I I Smalyukh T

J White Adv Mater 2013 25 5880

15

Figure 1 a Human iris closes in a bright environment and b opens in the dark c Schematic

drawing of the light-driven iris at its closed state where light illumination triggers petal

segments to bend inwards and to reduce the aperture size d The iris is open under weakno

light conditions when the anisotropic thermal expansion induces bending in the segments

thus opening the aperture

16

Figure 2 a Schematic drawing of a splay-bend LCE strip polymerized at an elevated

temperature (Tp gt TR) It remains flat when the environmental temperature Te = Tp and

spontaneously bends towards the homeotropic direction due to anisotropic thermal expansion

when the temperature is reduced to TR b The same strip polymerized at TR remains flat at TR

and bends to the opposite direction at an elevated temperature (Te gt TR) c The measured

bending angle α of LCE strips polymerized at different temperatures Tp The two LCE strips

remain flat at the polymerization temperature (α = 0 solid dots on the x-axis) and bend with

different angles when environmental temperature is changed (empty circles) Insets

photographs of the LCE strips (6times1times002 mm3) at different temperatures d Bending angle vs

temperature difference T between polymerization temperature and room temperature Inset

schematic drawing of the anisotropic thermal expansion bending direction with respect to the

splayed molecular orientation within the strip Error bars in c and b indicate standard

deviation for 3 independent measurements

17

Figure 3 Schematic of the soft iris fabrication process a A 20 microm cell is prepared by

placing together two glass slides coated with a homeotropic alignment layer on the top and

photoalignment layer at the bottom b LC monomer mixture is infiltrated into the cell and it

adapts a centrally-symmetric splayed alignment directed by c molecular orientation at the

top and bottom interfaces d Schematic of the photoalignment setup A linearly polarized 488

nm laser beam is converted into azimuthally polarized by using a polarization converter and

then projected onto the substrate coated with the photoalignment layer due to which the

azobenzene molecules within the photoalignment layer adapt radial distribution e Chemical

composition of the LC mixture used f Optical image of the polymerized LCE film cut out

with a circle and with 12 petal-like segments Inset cross-polarized image of the film before

cutting g Soft LCE iris in the open state when no light impinges on it h The iris closes upon

light illumination (470 nm 250 mW cm-2) Scale bars are 5 mm

18

Figure 4 a Measured transmission (black dots) and transmitted power (red dots) at different

input powers Light source 488 nm laser with circular polarization Beam diameter 12 mm

Error bars indicate standard deviation for 3 independent measurements Transmitted power

presents the average output power from 3 independent measurements and the dashed line

represents the case of 100 transmission through the device b Measured output power at

different times after switching on the light at t = 0 s Intensity 270 mWcm-2 Insets

photographs of the iris at different stages An optical filter is used to block wavelengths below

500 nm c Closure time for 12 petal segments at different illumination intensities The last

closed segment defines the complete closure time

19

The table of contents

Iris is a biological tissue that can change the pupil size to stabilize the light transmission into

the eye We report an iris-inspired soft device that can automatically adjust aperture size in

response to the change of incident light intensity The device is made of radially photoaligned

liquid crystal elastomer using anisotropic thermal expansion to realize the desired

photoactuation mode

Keywords iris biomimetic photoactuation liquid crystal elastomer azobenzene

Hao Zeng Owies M Wani Piotr Wasylczyk Radosław Kaczmarek Arri Priimagi

Self-regulating iris based on light-actuable liquid crystal elastomer

ToC figure

15

Figure 1 a Human iris closes in a bright environment and b opens in the dark c Schematic

drawing of the light-driven iris at its closed state where light illumination triggers petal

segments to bend inwards and to reduce the aperture size d The iris is open under weakno

light conditions when the anisotropic thermal expansion induces bending in the segments

thus opening the aperture

16

Figure 2 a Schematic drawing of a splay-bend LCE strip polymerized at an elevated

temperature (Tp gt TR) It remains flat when the environmental temperature Te = Tp and

spontaneously bends towards the homeotropic direction due to anisotropic thermal expansion

when the temperature is reduced to TR b The same strip polymerized at TR remains flat at TR

and bends to the opposite direction at an elevated temperature (Te gt TR) c The measured

bending angle α of LCE strips polymerized at different temperatures Tp The two LCE strips

remain flat at the polymerization temperature (α = 0 solid dots on the x-axis) and bend with

different angles when environmental temperature is changed (empty circles) Insets

photographs of the LCE strips (6times1times002 mm3) at different temperatures d Bending angle vs

temperature difference T between polymerization temperature and room temperature Inset

schematic drawing of the anisotropic thermal expansion bending direction with respect to the

splayed molecular orientation within the strip Error bars in c and b indicate standard

deviation for 3 independent measurements

17

Figure 3 Schematic of the soft iris fabrication process a A 20 microm cell is prepared by

placing together two glass slides coated with a homeotropic alignment layer on the top and

photoalignment layer at the bottom b LC monomer mixture is infiltrated into the cell and it

adapts a centrally-symmetric splayed alignment directed by c molecular orientation at the

top and bottom interfaces d Schematic of the photoalignment setup A linearly polarized 488

nm laser beam is converted into azimuthally polarized by using a polarization converter and

then projected onto the substrate coated with the photoalignment layer due to which the

azobenzene molecules within the photoalignment layer adapt radial distribution e Chemical

composition of the LC mixture used f Optical image of the polymerized LCE film cut out

with a circle and with 12 petal-like segments Inset cross-polarized image of the film before

cutting g Soft LCE iris in the open state when no light impinges on it h The iris closes upon

light illumination (470 nm 250 mW cm-2) Scale bars are 5 mm

18

Figure 4 a Measured transmission (black dots) and transmitted power (red dots) at different

input powers Light source 488 nm laser with circular polarization Beam diameter 12 mm

Error bars indicate standard deviation for 3 independent measurements Transmitted power

presents the average output power from 3 independent measurements and the dashed line

represents the case of 100 transmission through the device b Measured output power at

different times after switching on the light at t = 0 s Intensity 270 mWcm-2 Insets

photographs of the iris at different stages An optical filter is used to block wavelengths below

500 nm c Closure time for 12 petal segments at different illumination intensities The last

closed segment defines the complete closure time

19

The table of contents

Iris is a biological tissue that can change the pupil size to stabilize the light transmission into

the eye We report an iris-inspired soft device that can automatically adjust aperture size in

response to the change of incident light intensity The device is made of radially photoaligned

liquid crystal elastomer using anisotropic thermal expansion to realize the desired

photoactuation mode

Keywords iris biomimetic photoactuation liquid crystal elastomer azobenzene

Hao Zeng Owies M Wani Piotr Wasylczyk Radosław Kaczmarek Arri Priimagi

Self-regulating iris based on light-actuable liquid crystal elastomer

ToC figure

16

Figure 2 a Schematic drawing of a splay-bend LCE strip polymerized at an elevated

temperature (Tp gt TR) It remains flat when the environmental temperature Te = Tp and

spontaneously bends towards the homeotropic direction due to anisotropic thermal expansion

when the temperature is reduced to TR b The same strip polymerized at TR remains flat at TR

and bends to the opposite direction at an elevated temperature (Te gt TR) c The measured

bending angle α of LCE strips polymerized at different temperatures Tp The two LCE strips

remain flat at the polymerization temperature (α = 0 solid dots on the x-axis) and bend with

different angles when environmental temperature is changed (empty circles) Insets

photographs of the LCE strips (6times1times002 mm3) at different temperatures d Bending angle vs

temperature difference T between polymerization temperature and room temperature Inset

schematic drawing of the anisotropic thermal expansion bending direction with respect to the

splayed molecular orientation within the strip Error bars in c and b indicate standard

deviation for 3 independent measurements

17

Figure 3 Schematic of the soft iris fabrication process a A 20 microm cell is prepared by

placing together two glass slides coated with a homeotropic alignment layer on the top and

photoalignment layer at the bottom b LC monomer mixture is infiltrated into the cell and it

adapts a centrally-symmetric splayed alignment directed by c molecular orientation at the

top and bottom interfaces d Schematic of the photoalignment setup A linearly polarized 488

nm laser beam is converted into azimuthally polarized by using a polarization converter and

then projected onto the substrate coated with the photoalignment layer due to which the

azobenzene molecules within the photoalignment layer adapt radial distribution e Chemical

composition of the LC mixture used f Optical image of the polymerized LCE film cut out

with a circle and with 12 petal-like segments Inset cross-polarized image of the film before

cutting g Soft LCE iris in the open state when no light impinges on it h The iris closes upon

light illumination (470 nm 250 mW cm-2) Scale bars are 5 mm

18

Figure 4 a Measured transmission (black dots) and transmitted power (red dots) at different

input powers Light source 488 nm laser with circular polarization Beam diameter 12 mm

Error bars indicate standard deviation for 3 independent measurements Transmitted power

presents the average output power from 3 independent measurements and the dashed line

represents the case of 100 transmission through the device b Measured output power at

different times after switching on the light at t = 0 s Intensity 270 mWcm-2 Insets

photographs of the iris at different stages An optical filter is used to block wavelengths below

500 nm c Closure time for 12 petal segments at different illumination intensities The last

closed segment defines the complete closure time

19

The table of contents

Iris is a biological tissue that can change the pupil size to stabilize the light transmission into

the eye We report an iris-inspired soft device that can automatically adjust aperture size in

response to the change of incident light intensity The device is made of radially photoaligned

liquid crystal elastomer using anisotropic thermal expansion to realize the desired

photoactuation mode

Keywords iris biomimetic photoactuation liquid crystal elastomer azobenzene

Hao Zeng Owies M Wani Piotr Wasylczyk Radosław Kaczmarek Arri Priimagi

Self-regulating iris based on light-actuable liquid crystal elastomer

ToC figure

17

Figure 3 Schematic of the soft iris fabrication process a A 20 microm cell is prepared by

placing together two glass slides coated with a homeotropic alignment layer on the top and

photoalignment layer at the bottom b LC monomer mixture is infiltrated into the cell and it

adapts a centrally-symmetric splayed alignment directed by c molecular orientation at the

top and bottom interfaces d Schematic of the photoalignment setup A linearly polarized 488

nm laser beam is converted into azimuthally polarized by using a polarization converter and

then projected onto the substrate coated with the photoalignment layer due to which the

azobenzene molecules within the photoalignment layer adapt radial distribution e Chemical

composition of the LC mixture used f Optical image of the polymerized LCE film cut out

with a circle and with 12 petal-like segments Inset cross-polarized image of the film before

cutting g Soft LCE iris in the open state when no light impinges on it h The iris closes upon

light illumination (470 nm 250 mW cm-2) Scale bars are 5 mm

18

Figure 4 a Measured transmission (black dots) and transmitted power (red dots) at different

input powers Light source 488 nm laser with circular polarization Beam diameter 12 mm

Error bars indicate standard deviation for 3 independent measurements Transmitted power

presents the average output power from 3 independent measurements and the dashed line

represents the case of 100 transmission through the device b Measured output power at

different times after switching on the light at t = 0 s Intensity 270 mWcm-2 Insets

photographs of the iris at different stages An optical filter is used to block wavelengths below

500 nm c Closure time for 12 petal segments at different illumination intensities The last

closed segment defines the complete closure time

19

The table of contents

Iris is a biological tissue that can change the pupil size to stabilize the light transmission into

the eye We report an iris-inspired soft device that can automatically adjust aperture size in

response to the change of incident light intensity The device is made of radially photoaligned

liquid crystal elastomer using anisotropic thermal expansion to realize the desired

photoactuation mode

Keywords iris biomimetic photoactuation liquid crystal elastomer azobenzene

Hao Zeng Owies M Wani Piotr Wasylczyk Radosław Kaczmarek Arri Priimagi

Self-regulating iris based on light-actuable liquid crystal elastomer

ToC figure

18

Figure 4 a Measured transmission (black dots) and transmitted power (red dots) at different

input powers Light source 488 nm laser with circular polarization Beam diameter 12 mm

Error bars indicate standard deviation for 3 independent measurements Transmitted power

presents the average output power from 3 independent measurements and the dashed line

represents the case of 100 transmission through the device b Measured output power at

different times after switching on the light at t = 0 s Intensity 270 mWcm-2 Insets

photographs of the iris at different stages An optical filter is used to block wavelengths below

500 nm c Closure time for 12 petal segments at different illumination intensities The last

closed segment defines the complete closure time

19

The table of contents

Iris is a biological tissue that can change the pupil size to stabilize the light transmission into

the eye We report an iris-inspired soft device that can automatically adjust aperture size in

response to the change of incident light intensity The device is made of radially photoaligned

liquid crystal elastomer using anisotropic thermal expansion to realize the desired

photoactuation mode

Keywords iris biomimetic photoactuation liquid crystal elastomer azobenzene

Hao Zeng Owies M Wani Piotr Wasylczyk Radosław Kaczmarek Arri Priimagi

Self-regulating iris based on light-actuable liquid crystal elastomer

ToC figure

19

The table of contents

Iris is a biological tissue that can change the pupil size to stabilize the light transmission into

the eye We report an iris-inspired soft device that can automatically adjust aperture size in

response to the change of incident light intensity The device is made of radially photoaligned

liquid crystal elastomer using anisotropic thermal expansion to realize the desired

photoactuation mode

Keywords iris biomimetic photoactuation liquid crystal elastomer azobenzene

Hao Zeng Owies M Wani Piotr Wasylczyk Radosław Kaczmarek Arri Priimagi

Self-regulating iris based on light-actuable liquid crystal elastomer

ToC figure